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 e5ce0463..c2078a1d 100644
--- a/video-gen-api/app/services/private_portrait/ark_client.py
+++ b/video-gen-api/app/services/private_portrait/ark_client.py
@@ -149,7 +149,11 @@ class ArkPrivateAssetClient:
             )
         }
         service = Service(service_info, api_info)
-        raw = service.json(action.value, {}, payload)
+
+        # volcengine SDK 在签名阶段会对 request.body 做 hashlib.sha256(body)。
+        # Python 3 下 hashlib.sha256 只能接收 bytes/bytearray，不能接收 dict 或 str。
+        body = json.dumps(payload, ensure_ascii=False, separators=(",", ":")).encode("utf-8")
+        raw = service.json(action.value, {}, body)
 
         resp = self._normalize_response(raw)
         metadata = resp.get("ResponseMetadata") if isinstance(resp, dict) else None