diff --git a/vnpy/api/rest/rest_client.py b/vnpy/api/rest/rest_client.py
index 7bd4568e..646c894e 100644
--- a/vnpy/api/rest/rest_client.py
+++ b/vnpy/api/rest/rest_client.py
@@ -6,7 +6,7 @@ from datetime import datetime
 from enum import Enum
 from multiprocessing.dummy import Pool
 from queue import Empty, Queue
-from typing import Any, Callable, Optional
+from typing import Any, Callable, Optional, Union
 
 import requests
 
@@ -28,7 +28,7 @@ class Request(object):
         method: str,
         path: str,
         params: dict,
-        data: dict,
+        data: Union[dict, str, bytes],
         headers: dict,
         callback: Callable = None,
         on_failed: Callable = None,
@@ -140,7 +140,7 @@ class RestClient(object):
         path: str,
         callback: Callable,
         params: dict = None,
-        data: dict = None,
+        data: Union[dict, str, bytes] = None,
         headers: dict = None,
         on_failed: Callable = None,
         on_error: Callable = None,
@@ -152,7 +152,7 @@ class RestClient(object):
         :param path: 
         :param callback: callback function if 2xx status, type: (dict, Request)
         :param params: dict for query string
-        :param data: dict for body
+        :param data: Http body. If it is a dict, it will be converted to form-data. Otherwise, it will be converted to bytes.
         :param headers: dict for headers
         :param on_failed: callback function if Non-2xx status, type, type: (code, dict, Request)
         :param on_error: callback function when catching Python exception, type: (etype, evalue, tb, Request)