diff --git a/api.py b/api.py
index 949be41..1e2cfe9 100644
--- a/api.py
+++ b/api.py
@@ -103,6 +103,10 @@ async def put_chunk(transfer_id: str, idx: int, request: Request):
 async def complete(transfer_id: str):
     if tasks.get(transfer_id) is None:
         return JSONResponse(status_code=404, content={"error": "任务不存在"})
+    # 幂等: 已完成的任务直接返回已有 file_id (客户端超时重试/断线重连场景)
+    done_rec = db.get_file_by_transfer(transfer_id)
+    if done_rec is not None:
+        return {"status": "done", "file_id": done_rec["file_id"]}
     try:
         tasks.set_status(transfer_id, "assembling")
         merged = assembler.assemble(transfer_id)
diff --git a/db.py b/db.py
index 05ac569..3dc94c2 100644
--- a/db.py
+++ b/db.py
@@ -177,6 +177,15 @@ class ServerDB:
             )
         return file_id
 
+    def get_file_by_transfer(self, transfer_id: str) -> dict[str, Any] | None:
+        """按任务查已入库文件 (complete 幂等: 已完成直接返回)"""
+        with self._conn() as conn:
+            row = conn.execute(
+                "SELECT file_id, file_name, path, size FROM files WHERE transfer_id = ?",
+                (transfer_id,),
+            ).fetchone()
+        return dict(row) if row else None
+
     def list_files(self) -> list[dict[str, Any]]:
         """全部文件记录 (ls 端点)"""
         with self._conn() as conn:
diff --git a/tests/test_api.py b/tests/test_api.py
index 0159f7b..436c143 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -175,6 +175,13 @@ class ServerPipelineTest(unittest.TestCase):
         # 未知 file_id -> 404
         self.assertEqual(self.client.get("/api/files/nope").status_code, 404)
 
+        # complete 幂等: 重复 complete 返回同一个 file_id, 不新增入库记录
+        before = len(self.client.get("/api/files").json()["files"])
+        r = self.client.post(f"/api/transfer/{tid}/complete")
+        self.assertEqual(r.status_code, 200)
+        self.assertEqual(r.json()["file_id"], file_id)
+        self.assertEqual(len(self.client.get("/api/files").json()["files"]), before)
+
 
 if __name__ == "__main__":
     unittest.main()