$("#from_prov_id,#from_city_id").change(function () {
 var current_prov_id = $(this).val();
 var _this = this;
 $.ajax({
 type:'get',
 url:"{:url('getarea')}?id="+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="">選擇區（縣）</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="">選擇城市</option>');
 $.each(data.list1,function (index,item) {
 $("#from_city_id").append( '<option value="'+item.id+'">'+item.name+'</option>');
 });
 $("#from_area_id").append( '<option value="">選擇區（縣）</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="">選擇省份</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">江蘇</option>
 {/volist}
</select>
<select name="from_city_id" id="from_city_id" class="select">
 <option value="">選擇城市</option>
</select>
<select name="from_area_id" id="from_area_id" class="select">
 <option value="">選擇區縣</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]);
}