diff --git a/qcloud_cos/cos_client.py b/qcloud_cos/cos_client.py
index e3cbcf29..6a0bdcc3 100644
--- a/qcloud_cos/cos_client.py
+++ b/qcloud_cos/cos_client.py
@@ -46,7 +46,7 @@ def __init__(self, Appid=None, Region=None, Secret_id=None, Secret_key=None, Tok
         self._timeout = Timeout
 
         if Scheme is None:
-            Scheme = u'http'
+            Scheme = u'https'
         Scheme = to_unicode(Scheme)
         if(Scheme != u'http' and Scheme != u'https'):
             raise CosClientError('Scheme can be only set to http/https')
@@ -306,7 +306,7 @@ def get_presigned_download_url(self, Bucket, Key, Expired=300):
             )
         """
         url = self._conf.uri(bucket=Bucket, path=Key)
-        sign = self.get_auth(Method='GET', Bucket=Bucket, Key=Key, Expired=300)
+        sign = self.get_auth(Method='GET', Bucket=Bucket, Key=Key, Expired=Expired)
         url = url + '?sign=' + quote(sign)
         return url