diff --git a/src/BuildFlow.Host/Configuration/DatabaseInitializer.cs b/src/BuildFlow.Host/Configuration/DatabaseInitializer.cs
index 5041352..ebc678b 100644
--- a/src/BuildFlow.Host/Configuration/DatabaseInitializer.cs
+++ b/src/BuildFlow.Host/Configuration/DatabaseInitializer.cs
@@ -7,6 +7,7 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Diagnostics;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
+using MongoDB.Entities;
 using Workflow.Infrastructure.Persistence;
 using RagSeedData = BuildFlow.Infrastructure.Persistence.SeedData;
 using PlatformSeedData = BuildFlow.Platform.Infrastructure.Persistence.SeedData;
@@ -37,13 +38,19 @@ public static class DatabaseInitializer
         await MigrateAsync<ImDbContext>(sp, logger);
         await MigrateAsync<WorkflowDbContext>(sp, logger, suppressPendingChanges: true);
 
-        // 2. IM MongoDB 建索引（可选，Mongo 不可用时跳过）
+        // 2. IM MongoDB 建索引（MongoDB.Entities：用 DB.Index<MessageDocument> 创建两个复合索引）
+        //    两个复合索引支撑两种查询路径：按时间倒序拉取最新消息、按 ObjectId 游标分页上拉更早消息。
         await SafeRunAsync(
             async () =>
             {
-                var mongoContext = sp.GetService<MongoDbContext>();
-                if (mongoContext is not null)
-                    await mongoContext.CreateIndexesAsync();
+                await DB.Default.Index<MessageDocument>()
+                    .Key(m => m.ConversationId, KeyType.Ascending)
+                    .Key(m => m.CreatedAt, KeyType.Descending)
+                    .CreateAsync();
+                await DB.Default.Index<MessageDocument>()
+                    .Key(m => m.ConversationId, KeyType.Ascending)
+                    .Key(m => m.ID, KeyType.Descending)
+                    .CreateAsync();
             },
             logger,
             "IM MongoDB 建索引");
diff --git a/src/modules/im/Application/Chat/Handlers/GetMessageHistoryQueryHandler.cs b/src/modules/im/Application/Chat/Handlers/GetMessageHistoryQueryHandler.cs
index 0b01da6..83b9643 100644
--- a/src/modules/im/Application/Chat/Handlers/GetMessageHistoryQueryHandler.cs
+++ b/src/modules/im/Application/Chat/Handlers/GetMessageHistoryQueryHandler.cs
@@ -5,7 +5,7 @@ using IM.Domain.Common;
 using IM.Infrastructure.Mongo;
 using IM.Infrastructure.Persistence;
 using MediatR;
-using MongoDB.Driver;
+using MongoDB.Entities;
 
 namespace IM.Application.Chat.Handlers;
 
@@ -14,7 +14,6 @@ namespace IM.Application.Chat.Handlers;
 /// </summary>
 public class GetMessageHistoryQueryHandler(
     ImDbContext db,
-    MongoDbContext mongoDb,
     IUserInfoService userInfoService) : IRequestHandler<GetMessageHistoryQuery, MessageHistoryDto>
 {
     public async Task<MessageHistoryDto> Handle(GetMessageHistoryQuery request, CancellationToken ct)
@@ -26,21 +25,20 @@ public class GetMessageHistoryQueryHandler(
         if (!isParticipant)
             throw new UnauthorizedException("无权查看该会话的消息");
 
-        var filter = Builders<MessageDocument>.Filter.Eq(m => m.ConversationId, request.ConversationId);
+        var query = DB.Default.Find<MessageDocument>()
+            .Match(m => m.ConversationId == request.ConversationId);
 
-        // 游标条件：ObjectId 大致按时间单调递增，故 Lt(Id, BeforeId) 等价于"取该游标之前的消息"
+        // 游标条件：ObjectId 大致按时间单调递增，故 ID < BeforeId 等价于"取该游标之前的消息"
         if (!string.IsNullOrEmpty(request.BeforeId))
         {
-            var beforeFilter = Builders<MessageDocument>.Filter.Lt(m => m.Id, request.BeforeId);
-            filter = Builders<MessageDocument>.Filter.And(filter, beforeFilter);
+            query = query.Match(m => m.ID.CompareTo(request.BeforeId) < 0);
         }
 
         // Limit+1 技巧：多取一条用于判断是否还有更早的消息（HasMore），无需额外 COUNT 查询
-        var messages = await mongoDb.Messages
-            .Find(filter)
-            .SortByDescending(m => m.CreatedAt)
+        var messages = await query
+            .Sort(m => m.CreatedAt, Order.Descending)
             .Limit(request.Limit + 1)
-            .ToListAsync(ct);
+            .ExecuteAsync(ct);
 
         var hasMore = messages.Count > request.Limit;
         if (hasMore)
@@ -66,7 +64,7 @@ public class GetMessageHistoryQueryHandler(
             }
 
             return new MessageDto(
-                m.Id,
+                m.ID,
                 m.ConversationId,
                 m.SenderId,
                 m.SenderName,
diff --git a/src/modules/im/Application/Chat/Handlers/RecallMessageCommandHandler.cs b/src/modules/im/Application/Chat/Handlers/RecallMessageCommandHandler.cs
index 5a5f642..f89d39a 100644
--- a/src/modules/im/Application/Chat/Handlers/RecallMessageCommandHandler.cs
+++ b/src/modules/im/Application/Chat/Handlers/RecallMessageCommandHandler.cs
@@ -3,7 +3,7 @@ using IM.Application.Chat.Commands;
 using IM.Infrastructure.Mongo;
 using IM.Infrastructure.Persistence;
 using MediatR;
-using MongoDB.Driver;
+using MongoDB.Entities;
 
 namespace IM.Application.Chat.Handlers;
 
@@ -19,7 +19,7 @@ namespace IM.Application.Chat.Handlers;
 /// </list>
 /// 副作用：更新 Mongo 文档 IsRecalled=true；若撤回的是会话最后一条，更新会话摘要为"[消息已撤回]"。
 /// </remarks>
-public class RecallMessageCommandHandler(ImDbContext db, MongoDbContext mongoDb) : IRequestHandler<RecallMessageCommand, Unit>
+public class RecallMessageCommandHandler(ImDbContext db) : IRequestHandler<RecallMessageCommand, Unit>
 {
     /// <summary>发送者本人撤回的时间窗（分钟）。超时后本人不可撤回，仅管理员可。</summary>
     public const int RecallTimeWindowMinutes = 2;
@@ -27,10 +27,10 @@ public class RecallMessageCommandHandler(ImDbContext db, MongoDbContext mongoDb)
     public async Task<Unit> Handle(RecallMessageCommand request, CancellationToken ct)
     {
         // 查消息原文（用于校验发送者、创建时间）
-        var message = await mongoDb.Messages
-            .Find(m => m.Id == request.MessageId && m.ConversationId == request.ConversationId)
-            .FirstOrDefaultAsync(ct)
-            ?? throw new NotFoundException("消息不存在");
+        var messages = await DB.Default.Find<MessageDocument>()
+            .Match(m => m.ID == request.MessageId && m.ConversationId == request.ConversationId)
+            .ExecuteAsync(ct);
+        var message = messages.FirstOrDefault() ?? throw new NotFoundException("消息不存在");
 
         if (message.IsRecalled)
             throw new BusinessException("该消息已撤回");
@@ -48,14 +48,13 @@ public class RecallMessageCommandHandler(ImDbContext db, MongoDbContext mongoDb)
             throw new BusinessException($"消息发送超过 {RecallTimeWindowMinutes} 分钟，无法撤回");
         }
 
-        // 更新 Mongo：标记撤回
-        var update = Builders<MessageDocument>.Update
-            .Set(m => m.IsRecalled, true)
-            .Set(m => m.RecalledAt, DateTime.UtcNow)
-            .Set(m => m.RecalledBy, request.OperatorId);
-        await mongoDb.Messages.UpdateOneAsync(
-            Builders<MessageDocument>.Filter.Eq(m => m.Id, request.MessageId),
-            update, cancellationToken: ct);
+        // 更新 Mongo：标记撤回（按主键定位，逐字段 Modify，无需构造 Builders）
+        await DB.Default.Update<MessageDocument>()
+            .MatchID(request.MessageId)
+            .Modify(m => m.IsRecalled, true)
+            .Modify(m => m.RecalledAt, DateTime.UtcNow)
+            .Modify(m => m.RecalledBy, request.OperatorId)
+            .ExecuteAsync(ct);
 
         // 若撤回的是会话最后一条消息，更新摘要为撤回提示，避免列表显示已撤回内容
         var conversation = await db.Conversations.FirstOrDefaultAsync(c => c.Id == request.ConversationId, ct);
diff --git a/src/modules/im/Application/Chat/Handlers/SendMessageCommandHandler.cs b/src/modules/im/Application/Chat/Handlers/SendMessageCommandHandler.cs
index e3ab448..8fc0045 100644
--- a/src/modules/im/Application/Chat/Handlers/SendMessageCommandHandler.cs
+++ b/src/modules/im/Application/Chat/Handlers/SendMessageCommandHandler.cs
@@ -7,6 +7,7 @@ using IM.Domain.Enums;
 using IM.Infrastructure.Mongo;
 using IM.Infrastructure.Persistence;
 using MediatR;
+using MongoDB.Entities;
 
 namespace IM.Application.Chat.Handlers;
 
@@ -15,7 +16,6 @@ namespace IM.Application.Chat.Handlers;
 /// </summary>
 public class SendMessageCommandHandler(
     ImDbContext db,
-    MongoDbContext mongoDb,
     IUserInfoService userInfoService,
     IUnreadCounter unreadCounter,
     IOnlinePresenceService onlinePresence,
@@ -76,12 +76,12 @@ public class SendMessageCommandHandler(
             CreatedAt = DateTime.UtcNow
         };
 
-        // ① 写 MongoDB：消息正文持久化，InsertOne 后自动生成 ObjectId 作为消息主键
-        await mongoDb.Messages.InsertOneAsync(message, cancellationToken: ct);
+        // ① 写 MongoDB：消息正文持久化，SaveAsync 后自动生成 ObjectId 作为消息主键（ID）
+        await DB.Default.SaveAsync(message, ct);
 
         // ② 写 PostgreSQL：仅同步会话列表摘要与会话级"最后一条消息"指针（读扩散热数据）。
         //    注意：未读数累加已移出此事务（见下文 ③），避免大群发消息时的 PG 行锁竞争与写放大。
-        conversation.LastMessageId = message.Id;
+        conversation.LastMessageId = message.ID;
         conversation.LastMessageContent = BuildLastMessageContent(request.ContentType, request.Content, request.Metadata);
         conversation.LastMessageAt = DateTime.UtcNow;
 
@@ -125,7 +125,7 @@ public class SendMessageCommandHandler(
         await TryTriggerAiBotAsync(request, conversation, ct);
 
         return new MessageDto(
-            message.Id,
+            message.ID,
             message.ConversationId,
             message.SenderId,
             request.SenderName,
@@ -184,10 +184,10 @@ public class SendMessageCommandHandler(
                 ReplyToId = null,
                 CreatedAt = DateTime.UtcNow
             };
-            await mongoDb.Messages.InsertOneAsync(aiMessage, cancellationToken: ct);
+            await DB.Default.SaveAsync(aiMessage, ct);
 
             // 更新会话摘要
-            conversation.LastMessageId = aiMessage.Id;
+            conversation.LastMessageId = aiMessage.ID;
             conversation.LastMessageContent = $"[AI] {Truncate(answer, MaxLastMessageContentLength - 5)}";
             conversation.LastMessageAt = DateTime.UtcNow;
             await db.SaveChangesAsync(ct);
diff --git a/src/modules/im/Infrastructure/IMInfrastructureModule.cs b/src/modules/im/Infrastructure/IMInfrastructureModule.cs
index ba9f195..55b4233 100644
--- a/src/modules/im/Infrastructure/IMInfrastructureModule.cs
+++ b/src/modules/im/Infrastructure/IMInfrastructureModule.cs
@@ -7,6 +7,7 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using MongoDB.Driver;
+using MongoDB.Entities;
 using Volo.Abp.Modularity;
 
 namespace IM.Infrastructure;
@@ -33,15 +34,12 @@ public class IMInfrastructureModule : AbpModule
                    .AddInterceptors(sp.GetRequiredService<AuditInterceptor>()));
         services.AddScoped<AuditInterceptor>();
 
-        // MongoDB：消息正文。IMongoClient 单例复用连接池，MongoDbContext Scoped 保证每次请求独立上下文
-        services.AddSingleton<IMongoClient>(_ =>
-            new MongoClient(config.GetConnectionString("MongoDb")));
-        services.AddScoped(sp =>
-        {
-            var client = sp.GetRequiredService<IMongoClient>();
-            var dbName = MongoUrl.Create(config.GetConnectionString("MongoDb")!).DatabaseName;
-            return new Mongo.MongoDbContext(client, dbName);
-        });
+        // MongoDB（MongoDB.Entities）：消息正文。连接串由 ConnectionStrings:MongoDb 提供，
+        // 库名从连接串路径解析（与原 MongoUrl 逻辑一致）。
+        // 用静态 DB.InitAsync 一次性初始化，之后全局通过静态 DB/实体实例方法访问，无需 DI 注入。
+        var mongoConn = config.GetConnectionString("MongoDb")!;
+        var mongoDbName = MongoUrl.Create(mongoConn).DatabaseName!;
+        DB.InitAsync(mongoDbName, MongoClientSettings.FromConnectionString(mongoConn)).GetAwaiter().GetResult();
 
         services.AddImRedis(config);
 
diff --git a/src/modules/im/Infrastructure/Mongo/MessageDocument.cs b/src/modules/im/Infrastructure/Mongo/MessageDocument.cs
index 8246fb8..751d844 100644
--- a/src/modules/im/Infrastructure/Mongo/MessageDocument.cs
+++ b/src/modules/im/Infrastructure/Mongo/MessageDocument.cs
@@ -1,20 +1,18 @@
 using IM.Domain.Enums;
 using MongoDB.Bson;
 using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Entities;
 
 namespace IM.Infrastructure.Mongo;
 
 
 /// <summary>
 /// 消息 MongoDB 文档模型。IM 消息量级大、写频繁，用 Mongo 承载消息正文与元数据，支撑历史消息检索。
+/// 继承 <see cref="Entity"/>：主键 <c>ID</c>（string/ObjectId）由基类提供并自动生成，
+/// 替代原 <c>[BsonId] Id</c>。<c>[BsonRepresentation]</c> 特性全部保留，存量数据格式兼容。
 /// </summary>
-public class MessageDocument
+public class MessageDocument : Entity
 {
-    // ObjectId 作为主键，BsonRepresentation 使其在 C# 端表现为 string，便于游标透传
-    [BsonId]
-    [BsonRepresentation(BsonType.ObjectId)]
-    public string Id { get; set; } = default!;
-
     // Guid 以 String 形式存入 Mongo，保证与 PostgreSQL 中的 ConversationId/SenderId 一致可比
     [BsonRepresentation(BsonType.String)]
     public Guid ConversationId { get; set; }
diff --git a/src/modules/im/Infrastructure/Mongo/MongoDbContext.cs b/src/modules/im/Infrastructure/Mongo/MongoDbContext.cs
deleted file mode 100644
index 9ceca4d..0000000
--- a/src/modules/im/Infrastructure/Mongo/MongoDbContext.cs
+++ /dev/null
@@ -1,44 +0,0 @@
-using MongoDB.Driver;
-
-namespace IM.Infrastructure.Mongo;
-
-/// <summary>
-/// MongoDB 上下文，负责承载消息集合（高频追加写场景）。
-/// 与 PostgreSQL 的 <c>ImDbContext</c> 分工：消息正文落 MongoDB，会话/群组元数据落 PostgreSQL。
-/// </summary>
-public class MongoDbContext
-{
-    private readonly IMongoDatabase _database;
-
-    public MongoDbContext(IMongoClient mongoClient, string databaseName)
-    {
-        _database = mongoClient.GetDatabase(databaseName);
-    }
-
-    /// <summary>消息集合。每条文档对应一条聊天消息，主键为 MongoDB ObjectId。</summary>
-    public IMongoCollection<MessageDocument> Messages =>
-        _database.GetCollection<MessageDocument>("messages");
-
-    /// <summary>
-    /// 创建查询所需索引。Admin Host 启动时调用一次（幂等）。
-    /// </summary>
-    /// <remarks>
-    /// 两个复合索引分别支撑两种查询路径：
-    /// <list type="bullet">
-    /// <item>{ConversationId ASC, CreatedAt DESC} —— 按时间倒序拉取会话最新消息（默认进入页面）。</item>
-    /// <item>{ConversationId ASC, Id DESC} —— 游标分页按 ObjectId 倒序，支撑"上拉加载更早消息"。</item>
-    /// </list>
-    /// </remarks>
-    public async Task CreateIndexesAsync()
-    {
-        var indexKeys = Builders<MessageDocument>.IndexKeys
-            .Ascending(m => m.ConversationId)
-            .Descending(m => m.CreatedAt);
-        await Messages.Indexes.CreateOneAsync(new CreateIndexModel<MessageDocument>(indexKeys));
-
-        var cursorIndexKeys = Builders<MessageDocument>.IndexKeys
-            .Ascending(m => m.ConversationId)
-            .Descending(m => m.Id);
-        await Messages.Indexes.CreateOneAsync(new CreateIndexModel<MessageDocument>(cursorIndexKeys));
-    }
-}
diff --git a/src/modules/im/Infrastructure/Persistence/ImDbContext.cs b/src/modules/im/Infrastructure/Persistence/ImDbContext.cs
index 930b36e..9af3e29 100644
--- a/src/modules/im/Infrastructure/Persistence/ImDbContext.cs
+++ b/src/modules/im/Infrastructure/Persistence/ImDbContext.cs
@@ -7,7 +7,7 @@ namespace IM.Infrastructure.Persistence;
 
 /// <summary>
 /// IM 服务 PostgreSQL 上下文。管理会话、参与者、群组、群成员四张元数据表。
-/// 消息正文不在此处，存于 MongoDB（见 <see cref="Mongo.MongoDbContext"/>）。
+/// 消息正文不在此处，存于 MongoDB（见 <see cref="Mongo.MessageDocument"/>，经 MongoDB.Entities 访问）。
 /// </summary>
 public class ImDbContext : DbContext
 {