<select name="type" size="1" id="type">
    <option>請選擇類型</option>
     <foreach name='cate' item='v'>
         <option value="{$v['ca_id']}">{$v.ca_name}</option>
     </foreach>
 </select>
 標簽：
 <select name="lable" size="1" id="lables">
 </select>

  $('#type').click(function(){
            $(this).change(function(){
                var objectModel = {};
                var   value = $(this).val();
               var   type = $(this).attr('id');
                objectModel[type] =value;
                $.ajax({
                    cache:false,
                    type:"POST",
                    url:site.web+"lable",
                    dataType:"json",
                    data:objectModel,
                    timeout:30000,
                    error:function(){
                        alert(site.web+"lable");
                    },
                    success:function(data){
                        $("#lables").empty();
                        var count = data.length;
                        var i = 0;
                        var b="";
                           for(i=0;i<count;i++){
                               b+="<option value='"+data[i].ca_id+"'>"+data[i].ca_name+"</option>";
                           }
                        $("#lables").append(b);
                    }
                });
               });
        }
    );

//后臺ajax驗證
  $result = array();
  $cate =$_POST['type'];
  $result = M('cate')->where(array('ca_pid'=> $cate))->field('ca_id,ca_name')->select();
  $this->ajaxReturn($result,"JSON");

   //搜索結果為$result
   .....
   echo json_encode($result);