diff --git a/video-gen-api/app/services/private_portrait/ark_client.py b/video-gen-api/app/services/private_portrait/ark_client.py
index fe59f17e..e5ce0463 100644
--- a/video-gen-api/app/services/private_portrait/ark_client.py
+++ b/video-gen-api/app/services/private_portrait/ark_client.py
@@ -136,12 +136,20 @@ class ArkPrivateAssetClient:
             10,
             60,
         )
-        api_info = {action.value: ApiInfo("POST", "/", f"Action={action.value}&Version={ARK_PRIVATE_PORTRAIT_VERSION}", {}, {})}
+        # volcengine SDK 的 ApiInfo.query 必须是 dict，不能传 "Action=xxx&Version=xxx" 字符串。
+        # 否则 Service.merge() 会按 dict 下标读取字符串，触发：
+        # TypeError: string indices must be integers, not 'str'。
+        api_info = {
+            action.value: ApiInfo(
+                "POST",
+                "/",
+                {"Action": action.value, "Version": ARK_PRIVATE_PORTRAIT_VERSION},
+                {},
+                {},
+            )
+        }
         service = Service(service_info, api_info)
-        try:
-            raw = service.json(action.value, {}, json.dumps(payload, ensure_ascii=False, separators=(",", ":")))
-        except TypeError:
-            raw = service.json(action.value, {}, payload)
+        raw = service.json(action.value, {}, payload)
 
         resp = self._normalize_response(raw)
         metadata = resp.get("ResponseMetadata") if isinstance(resp, dict) else None