diff --git a/src/modules/file/Application/Common/FileSizeBackfillHelper.cs b/src/modules/file/Application/Common/FileSizeBackfillHelper.cs
new file mode 100644
index 0000000..043b64e
--- /dev/null
+++ b/src/modules/file/Application/Common/FileSizeBackfillHelper.cs
@@ -0,0 +1,52 @@
+using File.Domain.Entities;
+using File.Infrastructure.Persistence;
+using File.Infrastructure.Storage;
+
+namespace File.Application.Common;
+
+/// <summary>
+/// 文件大小回填助手：对 Size==0 的 FileMeta，从 S3 HeadObject 取真实 ContentLength 写回。
+/// 用于读路径（列表加载）惰性修复早期未记录大小的旧数据——首次加载即回填并持久化，
+/// 后续加载直接命中 DB 不再访问 S3。对象缺失或空文件保留 Size=0；单个失败不中断整体。
+/// </summary>
+public static class FileSizeBackfillHelper
+{
+    public static async Task BackfillAsync(
+        FileDbContext db,
+        IS3StorageService s3,
+        IReadOnlyList<FileMeta> files,
+        CancellationToken ct)
+    {
+        var stale = files.Where(f => f.Size == 0).ToList();
+        if (stale.Count == 0)
+        {
+            return;
+        }
+
+        var updated = 0;
+        foreach (var file in stale)
+        {
+            try
+            {
+                var size = await s3.GetObjectSizeAsync(file.S3Bucket, file.S3Key, ct);
+                if (size is null || size <= 0)
+                {
+                    // 对象缺失（已被外部清理）或空文件：保留 Size=0
+                    continue;
+                }
+
+                file.Size = size.Value;
+                updated++;
+            }
+            catch
+            {
+                // 单个对象查询失败不中断整体回填，保留 Size=0 下次可重试
+            }
+        }
+
+        if (updated > 0)
+        {
+            await db.SaveChangesAsync(ct);
+        }
+    }
+}
diff --git a/src/modules/file/Application/Common/FolderSizeCalculator.cs b/src/modules/file/Application/Common/FolderSizeCalculator.cs
new file mode 100644
index 0000000..10925f8
--- /dev/null
+++ b/src/modules/file/Application/Common/FolderSizeCalculator.cs
@@ -0,0 +1,57 @@
+using File.Domain.Entities;
+
+namespace File.Application.Common;
+
+/// <summary>
+/// 文件夹递归大小计算器：给定全部文件夹 + 每个文件夹的直接文件总大小，
+/// 用后序 DFS 算出每个文件夹的递归总大小（自身文件 + 所有子孙文件夹文件）。
+/// 用于 getFolderTree 在返回前一次性算出所有文件夹的 Size。
+/// </summary>
+public static class FolderSizeCalculator
+{
+    public static Dictionary<Guid, long> Compute(
+        IReadOnlyList<Folder> folders,
+        IReadOnlyDictionary<Guid, long> filesSizeByFolder)
+    {
+        // parentId → 子文件夹 id 列表（根文件夹 parentId=null 归到 Guid.Empty 桶，不会被当文件夹遍历）
+        var childrenByParent = folders
+            .GroupBy(f => f.ParentId ?? Guid.Empty)
+            .ToDictionary(g => g.Key, g => g.Select(f => f.Id).ToList());
+
+        var result = new Dictionary<Guid, long>(folders.Count);
+        var inStack = new HashSet<Guid>(); // 防环（理论上 ParentId 不该成环，保险起见）
+
+        long Dfs(Guid id)
+        {
+            if (result.TryGetValue(id, out var cached))
+            {
+                return cached;
+            }
+
+            if (!inStack.Add(id))
+            {
+                return 0; // 检测到环，断开避免死循环
+            }
+
+            var sum = filesSizeByFolder.GetValueOrDefault(id, 0L);
+            if (childrenByParent.TryGetValue(id, out var children))
+            {
+                foreach (var childId in children)
+                {
+                    sum += Dfs(childId);
+                }
+            }
+
+            inStack.Remove(id);
+            result[id] = sum;
+            return sum;
+        }
+
+        foreach (var folder in folders)
+        {
+            Dfs(folder.Id);
+        }
+
+        return result;
+    }
+}
diff --git a/src/modules/file/Application/Features/Folders/Commands/FolderMapper.cs b/src/modules/file/Application/Features/Folders/Commands/FolderMapper.cs
index b78d288..0642097 100644
--- a/src/modules/file/Application/Features/Folders/Commands/FolderMapper.cs
+++ b/src/modules/file/Application/Features/Folders/Commands/FolderMapper.cs
@@ -6,5 +6,5 @@ namespace File.Application.Features.Folders.Commands;
 /// <summary>Folder ↔ FolderDto 映射工具。</summary>
 internal static class FolderMapper
 {
-    public static FolderDto ToDto(Folder f) => new(f.Id, f.ParentId, f.Name, f.OwnerId, f.CreatedAt, f.UpdatedAt);
+    public static FolderDto ToDto(Folder f, long size = 0) => new(f.Id, f.ParentId, f.Name, f.OwnerId, f.CreatedAt, f.UpdatedAt, size);
 }
diff --git a/src/modules/file/Application/Features/Folders/DTOs/FolderDtos.cs b/src/modules/file/Application/Features/Folders/DTOs/FolderDtos.cs
index adf3bed..3b3c741 100644
--- a/src/modules/file/Application/Features/Folders/DTOs/FolderDtos.cs
+++ b/src/modules/file/Application/Features/Folders/DTOs/FolderDtos.cs
@@ -3,5 +3,5 @@
 namespace File.Application.Features.Folders.DTOs;
 
 
-/// <summary>文件夹 DTO。</summary>
-public record FolderDto(Guid Id, Guid? ParentId, string Name, string OwnerId, DateTime CreatedAt, DateTime UpdatedAt);
+/// <summary>文件夹 DTO。Size = 该文件夹下所有文件递归汇总的字节大小（由 getFolderTree 计算）。</summary>
+public record FolderDto(Guid Id, Guid? ParentId, string Name, string OwnerId, DateTime CreatedAt, DateTime UpdatedAt, long Size = 0);
diff --git a/src/modules/file/Application/Features/Folders/Handlers/GetFolderQueryHandler.cs b/src/modules/file/Application/Features/Folders/Handlers/GetFolderQueryHandler.cs
index 233aaa5..5329937 100644
--- a/src/modules/file/Application/Features/Folders/Handlers/GetFolderQueryHandler.cs
+++ b/src/modules/file/Application/Features/Folders/Handlers/GetFolderQueryHandler.cs
@@ -1,15 +1,23 @@
 using BuildFlow.Shared.Domain.Common;
+using File.Application.Common;
 using File.Application.Features.Folders.Commands;
 using File.Application.Features.Folders.DTOs;
 using File.Application.Features.Folders.Queries;
 using File.Infrastructure.Persistence;
+using File.Infrastructure.Storage;
 using MediatR;
 
 namespace File.Application.Features.Folders.Handlers;
 
-/// <summary>查询单文件夹处理器。校验归属后聚合子文件夹与文件清单。</summary>
-public class GetFolderQueryHandler(FileDbContext db, ICurrentUserContext currentUser)
-    : IRequestHandler<GetFolderQuery, FolderWithChildrenDto?>
+/// <summary>
+/// 查询单文件夹处理器。校验归属后聚合子文件夹与文件清单。
+/// 加载时对 Size==0 的旧文件记录惰性回填（从 S3 HeadObject 取真实大小写回），
+/// 这样移动文件进文件夹后再加载也会自动补全大小。
+/// </summary>
+public class GetFolderQueryHandler(
+    FileDbContext db,
+    ICurrentUserContext currentUser,
+    IS3StorageService s3) : IRequestHandler<GetFolderQuery, FolderWithChildrenDto?>
 {
     public async Task<FolderWithChildrenDto?> Handle(GetFolderQuery req, CancellationToken ct)
     {
@@ -23,9 +31,11 @@ public class GetFolderQueryHandler(FileDbContext db, ICurrentUserContext current
             .OrderBy(f => f.Name)
             .ToListAsync(ct);
 
+        await FileSizeBackfillHelper.BackfillAsync(db, s3, files, ct);
+
         return new FolderWithChildrenDto(
             FolderMapper.ToDto(folder),
-            subFolders.Select(FolderMapper.ToDto).ToList(),
+            subFolders.Select(f => FolderMapper.ToDto(f)).ToList(),
             files.Select(f => new FileMetaDto(f.Id, f.FolderId, f.Name, f.S3Key, f.S3Bucket,
                 f.Size, f.ContentType, f.OwnerId, f.CreatedAt, f.UpdatedAt)).ToList());
     }
diff --git a/src/modules/file/Application/Features/Folders/Handlers/GetFolderTreeQueryHandler.cs b/src/modules/file/Application/Features/Folders/Handlers/GetFolderTreeQueryHandler.cs
index 8e0a47a..fc30c6d 100644
--- a/src/modules/file/Application/Features/Folders/Handlers/GetFolderTreeQueryHandler.cs
+++ b/src/modules/file/Application/Features/Folders/Handlers/GetFolderTreeQueryHandler.cs
@@ -1,20 +1,43 @@
 using BuildFlow.Shared.Domain.Common;
+using File.Application.Common;
 using File.Application.Features.Folders.Commands;
 using File.Application.Features.Folders.DTOs;
 using File.Application.Features.Folders.Queries;
 using File.Infrastructure.Persistence;
+using File.Infrastructure.Storage;
 using MediatR;
 
 namespace File.Application.Features.Folders.Handlers;
 
-/// <summary>文件夹树查询处理器。返回当前用户范围内的全部文件夹。</summary>
-public class GetFolderTreeQueryHandler(FileDbContext db, ICurrentUserContext currentUser)
-    : IRequestHandler<GetFolderTreeQuery, List<FolderDto>>
+/// <summary>
+/// 文件夹树查询处理器。返回当前用户范围内的全部文件夹。
+/// 同时计算每个文件夹的递归总大小（自身文件 + 所有子孙文件夹文件），写入 FolderDto.Size。
+/// 计算前先回填历史 Size=0 的文件，保证汇总准确。
+/// </summary>
+public class GetFolderTreeQueryHandler(
+    FileDbContext db,
+    ICurrentUserContext currentUser,
+    IS3StorageService s3) : IRequestHandler<GetFolderTreeQuery, List<FolderDto>>
 {
     public async Task<List<FolderDto>> Handle(GetFolderTreeQuery req, CancellationToken ct)
     {
         var owner = currentUser.GetRequiredUserId();
+
         var folders = await db.Folders.Where(f => f.OwnerId == owner).ToListAsync(ct);
-        return folders.Select(FolderMapper.ToDto).ToList();
+
+        // 加载该用户全部文件：先回填历史 Size=0 记录（保证文件夹大小准确），再按文件夹汇总
+        var files = await db.Files.Where(f => f.OwnerId == owner).ToListAsync(ct);
+        await FileSizeBackfillHelper.BackfillAsync(db, s3, files, ct);
+
+        var filesSizeByFolder = files
+            .Where(f => f.FolderId.HasValue)
+            .GroupBy(f => f.FolderId!.Value)
+            .ToDictionary(g => g.Key, g => g.Sum(f => f.Size));
+
+        var sizes = FolderSizeCalculator.Compute(folders, filesSizeByFolder);
+
+        return folders
+            .Select(f => FolderMapper.ToDto(f, sizes.GetValueOrDefault(f.Id, 0L)))
+            .ToList();
     }
 }
diff --git a/src/modules/file/Application/Features/Folders/Handlers/GetRootFilesQueryHandler.cs b/src/modules/file/Application/Features/Folders/Handlers/GetRootFilesQueryHandler.cs
index 140c025..dfbb6dc 100644
--- a/src/modules/file/Application/Features/Folders/Handlers/GetRootFilesQueryHandler.cs
+++ b/src/modules/file/Application/Features/Folders/Handlers/GetRootFilesQueryHandler.cs
@@ -1,7 +1,9 @@
 using BuildFlow.Shared.Domain.Common;
+using File.Application.Common;
 using File.Application.Features.Folders.DTOs;
 using File.Application.Features.Folders.Queries;
 using File.Infrastructure.Persistence;
+using File.Infrastructure.Storage;
 using MediatR;
 using Microsoft.EntityFrameworkCore;
 
@@ -10,9 +12,12 @@ namespace File.Application.Features.Folders.Handlers;
 /// <summary>
 /// 查询根级文件处理器。返回当前用户名下 FolderId=null 的所有 FileMeta（按名称排序）。
 /// 与 GetFolderQueryHandler 配合，使根目录与子文件夹共享同一数据源（DB），而非 S3 列表。
+/// 加载时对 Size==0 的旧记录惰性回填（从 S3 HeadObject 取真实大小写回）。
 /// </summary>
-public class GetRootFilesQueryHandler(FileDbContext db, ICurrentUserContext currentUser)
-    : IRequestHandler<GetRootFilesQuery, List<FileMetaDto>>
+public class GetRootFilesQueryHandler(
+    FileDbContext db,
+    ICurrentUserContext currentUser,
+    IS3StorageService s3) : IRequestHandler<GetRootFilesQuery, List<FileMetaDto>>
 {
     public async Task<List<FileMetaDto>> Handle(GetRootFilesQuery req, CancellationToken ct)
     {
@@ -22,6 +27,8 @@ public class GetRootFilesQueryHandler(FileDbContext db, ICurrentUserContext curr
             .OrderBy(f => f.Name)
             .ToListAsync(ct);
 
+        await FileSizeBackfillHelper.BackfillAsync(db, s3, files, ct);
+
         return files.Select(f => new FileMetaDto(f.Id, f.FolderId, f.Name, f.S3Key, f.S3Bucket,
             f.Size, f.ContentType, f.OwnerId, f.CreatedAt, f.UpdatedAt)).ToList();
     }
diff --git a/src/modules/file/Infrastructure/Storage/IS3StorageService.cs b/src/modules/file/Infrastructure/Storage/IS3StorageService.cs
index 1b49f62..1af2376 100644
--- a/src/modules/file/Infrastructure/Storage/IS3StorageService.cs
+++ b/src/modules/file/Infrastructure/Storage/IS3StorageService.cs
@@ -26,6 +26,12 @@ public interface IS3StorageService
     /// </summary>
     Task<bool> ObjectExistsAsync(string bucket, string key, CancellationToken ct);
 
+    /// <summary>
+    /// 取对象字节大小（HeadObject → ContentLength）。对象不存在返回 null。
+    /// 用于回填历史 FileMeta.Size=0 的记录（早期未记录大小的旧数据）。
+    /// </summary>
+    Task<long?> GetObjectSizeAsync(string bucket, string key, CancellationToken ct);
+
     // 分片上传
     Task<string> CreateMultipartUploadAsync(string bucket, string key, CancellationToken ct);
     Task<string> UploadPartAsync(string bucket, string key, string uploadId, int partNumber, Stream data, CancellationToken ct);
diff --git a/src/modules/file/Infrastructure/Storage/S3StorageService.cs b/src/modules/file/Infrastructure/Storage/S3StorageService.cs
index 2190153..39cd276 100644
--- a/src/modules/file/Infrastructure/Storage/S3StorageService.cs
+++ b/src/modules/file/Infrastructure/Storage/S3StorageService.cs
@@ -166,6 +166,21 @@ public class S3StorageService : IS3StorageService
         catch (Exception ex) { throw Wrap(ex); }
     }
 
+    public async Task<long?> GetObjectSizeAsync(string bucket, string key, CancellationToken ct)
+    {
+        try
+        {
+            // HeadObject 取 ContentLength；不存在抛 NotFound → 返回 null
+            var resp = await _client.GetObjectMetadataAsync(bucket, key, ct);
+            return resp?.ContentLength;
+        }
+        catch (Amazon.S3.AmazonS3Exception ex) when (ex.StatusCode == System.Net.HttpStatusCode.NotFound)
+        {
+            return null;
+        }
+        catch (Exception ex) { throw Wrap(ex); }
+    }
+
     public async Task<string> CreateMultipartUploadAsync(string bucket, string key, CancellationToken ct)
     {
         try