问题解决了！原来网页进行了Gzip压缩，解码就好了！

           string html = "";
            string txtUri = "https://www.";
            HttpWebRequest req = WebRequest.Create(txtUri) as HttpWebRequest;
            HttpWebResponse res = req.GetResponse() as HttpWebResponse;
            Stream s = res.GetResponseStream();
            if (res.StatusCode == HttpStatusCode.OK)
            {
                using (s) //using表示回收使用的内存
                {
                    using (StreamReader sr = new StreamReader(s, Encoding.GetEncoding("gb2312"))) //UTF-8 Unicode defaule这些全都试过了
                    {
                        html = sr.ReadToEnd();
                        sr.Close();
                    }
                    s.Close();
                }
            }