$.ajax({type:'POST',
  url:'b.php',
  data:'',
  timeout:5000,
  success:function(){
  }
})

<script src = "jquery-1.4.4.js"></script>
<script>
var xhr = $.ajax({type:'POST',
  url:'b.php',
  data:'',
  success:function(){
    alert('ok');
  }
})
alert(xhr);
console.log(xhr);
</script>
<button id="song">abort</button>
<script>
$(function(){
  $("#song").click(function(){
    alert('click');
    xhr.abort();
  })
})
</script>

xmlHttp.open("POST","theUrl",true);
xmlHttp.onreadystatechange=function(){
  ...//得到响应之后的操作
}
xmlHttp.send();
//设置8秒钟后检查xmlHttp对象所发送的数据是否得到响应.
setTimeout("CheckRequest()","8000");
function CheckRequest(){
  //为4时代表请求完成了  
  if(xmlHttp.readyState!=4){
    alert('响应超时');
    //关闭请求
    xmlHttp.close();
  }
}