$("#from_prov_id,#from_city_id").change(function () {
    var current_prov_id = $(this).val();
    var _this = this;
    $.ajax({
      type:'get',
      url:"{:url('getarea')}"+current_prov_id,
      success:function (data) {
        var data = JSON.parse(data);
        if (data.list2==''){
          $('#from_area_id option').remove();
          $("#from_area_id").append( '<option value="">选择区（县）</option>');
          $.each(data.list1,function (index,item) {
            $("#from_area_id").append( '<option value="'+item.id+'">'+item.name+'</option>');
          });
        }else{
          $("#from_city_id option").remove();
          $('#from_area_id option').remove();
          $("#from_city_id").append( '<option value="">选择城市</option>');
          $.each(data.list1,function (index,item) {
            $("#from_city_id").append( '<option value="'+item.id+'">'+item.name+'</option>');
          });
          $("#from_area_id").append( '<option value="">选择区（县）</option>');
          $.each(data.list2,function (index,item) {
            $("#from_area_id").append( '<option value="'+item.id+'">'+item.name+'</option>');
          });
        }
      }
    })
});

<select name="from_prov_id" id="from_prov_id" class="select">
    <option value="">选择省份</option>
      <option value="id">北京</option>
      <option value="id">上海</option>
      <option value="id">浙江</option>
      <option value="id">四川</option>
      <option value="id">山西</option>
      <option value="id">江苏</option>
    {/volist}
</select>
<select name="from_city_id" id="from_city_id" class="select">
    <option value="">选择城市</option>
</select>
<select name="from_area_id" id="from_area_id" class="select">
    <option value="">选择区县</option>
</select>

public function area(){
    $area = new Area();
    $id = input('id');
    $list = $area->getList(['pid'=>$id]);
    $list2 = $area->getList(['pid'=>$list[0]['id']]);
    return json_encode(['list1'=>$list,'list2'=>$list2]);
}