ajax 中的status为什么=500啊？？急 - .NET技术 / C#

<script type="text/javascript" language="javascript">
  function excuAjax(url, funName, param) {
  var xmlhttp;
  if (window.ActiveXObject) {
  xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  } else {
  xmlhttp = new XMLHttpRequest();
  }
  xmlhttp.open("GET", url, true);
  xmlhttp.onreadystatechange = function() {
  if (xmlhttp.readyState == 4) {
  if (xmlhttp.status == 200) {
  var xml = xmlhttp.responseXML;
  funName(xml);
  }
  }
  }
  xmlhttp.setRequestHeader("Content-Type", "application/JSON");
  xmlhttp.send(param);

  }
  function getProject() {
  var url = "http://localhost:3399/Web/WebService.asmx/HelloWorld";
  excuAjax(url, callStr, null);
  }
  function callStr(rs) {
  alert(rs);
  }
  function callProject(rs) {
  var projects = document.getElementById("s_Project");