1.添加一个SetHeaderValue方法：

        public static void SetHeaderValue(WebHeaderCollection header, string name, string value)        {            var property = typeof(WebHeaderCollection).GetProperty("InnerCollection",BindingFlags.Instance | BindingFlags.NonPublic);            if (property != null)            {                var collection = property.GetValue(header, null) as NameValueCollection;                collection[name] = value;            }        }

2.调用这个方法设置HttpHeader头：

      HttpWebRequest rq = (HttpWebRequest)WebRequest.Create("Url");                rq.Method = "GET";                SetHeaderValue(rq.Headers, "Host", "127.0.0.1");                SetHeaderValue(rq.Headers, "Connection", "keep-alive");                SetHeaderValue(rq.Headers, "Accept", "*/*");                SetHeaderValue(rq.Headers, "X-Requested-With", "XMLHttpRequest");                SetHeaderValue(rq.Headers, "User-Agent", "...");                SetHeaderValue(rq.Headers, "Referer", "http://127.0.0.1/index.php?m=Index&a=indexs");                SetHeaderValue(rq.Headers, "Accept-Encoding", "gzip, deflate");                SetHeaderValue(rq.Headers, "Accept-Language", "1.5");                SetHeaderValue(rq.Headers, "Cookie", "This is Cookie");                 HttpWebResponse resp = (HttpWebResponse)rq.GetResponse();                 using (Stream stream = resp.GetResponseStream())                {                    StreamReader reader =new StreamReader(stream,Encoding.Default);                    string responseString = reader.ReadToEnd();                    MessageBox.Show(responseString);                }

3.可以通过抓包软件（如SRSniffer）查看到设置的报头.