string str="{\"msgType\":1001,\"msgstring\":\"信息\"}";//这里是你的json带有反斜杠的
HttpResponseMessage result = new HttpResponseMessage { Content = new StringContent(str, Encoding.GetEncoding("UTF-8"), "application/json") };//这里是去掉反斜杠再放回出去，json就只剩下双引号。
return result;