!DOCTYPE html>
html lang="en">
head>
  meta charset="UTF-8">
  title>Title/title>
  script src="../wenjian/jquery-2.2.3.min.js">/script>
/head>
body>
select id="sheng">
  option>请选择/option>
/select>
select id="shi">
  option >请选择/option>
/select>
select id="qu">
  option >请选择/option>
/select>
/body>
/html>
script>
  $.ajax({
    data: {parent_id: 0}, //发送的数据
    dataType: "json", //返回值的类型 text为string型
    type: 'post',  //发送请求的方法(get)
    url: 'sheng_l.php',//发送请求的地址
    success: function (data) {//发送成功时的回调函数
  //      console.log(data);
      for (var i in data) {
        $("#sheng").append("option value='"+ data[i][2] +"'>" + data[i][1] +"/option>")
      }
    }
  })
  $(document).ready(function () {
    $("#sheng").change(function () {
      $("#shi").get(0).options.length= 1;
//      $("#qu").get(0).options.length= 1;
      var data = $("#sheng").find("option:selected").val();
      $.ajax({
        data:{parent_id:data},
        type:"post",
        dataType:"json",
        url:"sheng_l.php",
        success:function (data) {
          for(var i in data){
            $("#shi").append("option value='" + data[i][2] +"'>" + data[i][1] +"/option>")
          }
        }
      })
    })
  })
  $(document).ready(function () {
    $("#shi").change(function () {
      $("#qu").get(0).options.length= 1;
      var data = $("#shi").find("option:selected").val();
      $.ajax({
        data:{parent_id:data},
        type:"post",
        dataType:"json",
        url:"sheng_l.php",
        success:function (data) {
          for (var i in data){
            $("#qu").append("option value='" +data[i][2] +"'>" +data[i][1] +"/option>")
          }
        }
      })
    })
  })

?php
/**
 * Created by fcc
 * User: Administrator
 * Date: 2017/10/29
 * Time: 20:56
 */
require_once "../wenjian/DBDA.class.php";
$db = new DBDA();
$sql = "select * from region WHERE father_id = {$_POST['parent_id']}";
$result = $db->Query($sql);
echo json_encode($result);