string url = "http://localhost:1857/Test.asmx/HelloWorld";
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            //request.Credentials = CredentialCache.DefaultCredentials;
            request.Timeout = 10000;
            byte[] data = Encoding.UTF8.GetBytes(Pars);//参数
            request.ContentLength = data.Length;
            Stream writer = request.GetRequestStream();
            writer.Write(data, 0, data.Length);
            writer.Close();

            WebResponse response = request.GetResponse();
            StreamReader sr = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
            String retXml = sr.ReadToEnd();
            sr.Close();
            XmlDocument doc = new XmlDocument();
            doc.LoadXml(retXml);

            return doc;

 /// <summary>
    /// Test 的摘要说明
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // 若要允许使用 ASP.NET AJAX 从脚本中调用此 Web 服务，请取消对下行的注释。
    // [System.Web.Script.Services.ScriptService]
    public class Test : System.Web.Services.WebService
    {

        [WebMethod]
        public string HelloWorld(string xml)
        {
            return "Hello World";
        }
    }

 <webServices>
      <protocols>
        <add name="HttpGet"/>
        <add name="HttpPost"/>
      </protocols>
    </webServices>