diff --git a/backend/scripts/backfill_affiliations.py b/backend/scripts/backfill_affiliations.py
index 3c23736..f5be5b6 100644
--- a/backend/scripts/backfill_affiliations.py
+++ b/backend/scripts/backfill_affiliations.py
@@ -32,7 +32,10 @@ _VACUUM_EVERY = 10  # 每 10 批执行一次 VACUUM，回收死元组空间，
 
 async def main() -> None:
     settings = Settings()
-    conn = await asyncpg.connect(settings.DATABASE_URL_SYNC)
+    # 容器里只有 DATABASE_URL（async 驱动），DATABASE_URL_SYNC 没设会落到 localhost 默认值
+    # → 从 DATABASE_URL 推导：去掉 SQLAlchemy 驱动名即是 asyncpg 可用的 URL
+    sync_url = settings.DATABASE_URL.replace("+asyncpg", "")
+    conn = await asyncpg.connect(sync_url)
     try:
         await conn.execute("SET max_parallel_workers_per_gather = 0")
         remaining = await conn.fetchval(