String Uid = "网建短信通用户名";
String key = "接口短信密钥";
String smsMob = "13800000000";
String smsText = "验证码：8888";
string url = "http://gbk.api.smschinese.cn/";
string post = string.Format("Uid={0}&key={1}&smsMob={2}&smsText={3}", Uid, key, smsMob, smsText);
Response.Write(PostData(url, post));

public static string PostData(string posturl, string postData)
{
    //ServicePointManager.ServerCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;
    Stream outstream = null;
    Stream instream = null;
    StreamReader sr = null;
    HttpWebResponse response = null;
    HttpWebRequest request = null;
    Encoding encoding = Encoding.UTF8;
    byte[] data = encoding.GetBytes(postData);
    // 准备请求...
    try
    {
        //ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;
        //ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;
        // 设置参数

        //ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
        request = WebRequest.Create(posturl) as HttpWebRequest;
        CookieContainer cookieContainer = new CookieContainer();
        request.CookieContainer = cookieContainer;
        request.AllowAutoRedirect = true;
        request.Method = "POST";

        request.ContentType = "application/x-www-form-urlencoded";
        request.ContentLength = data.Length;
        outstream = request.GetRequestStream();
        outstream.Write(data, 0, data.Length);
        outstream.Close();
        //发送请求并获取相应回应数据
        response = request.GetResponse() as HttpWebResponse;
        //直到request.GetResponse()程序才开始向目标网页发送Post请求
        instream = response.GetResponseStream();
        sr = new StreamReader(instream, encoding);
        //返回结果网页（html）代码
        string content = sr.ReadToEnd();
        string err = string.Empty;
        return content;
    }
    catch (Exception ex)
    {
        return ex.Message;
    }
}