//省市县数据格式
var province_city_county_data=[
  {
    province:"四川",
    city:[
      {
        cityName:"成都",
        county:["成都市","崇州市","金堂县"]
      },
      {
        cityName:"南充",
        county:["仪陇县","南部县","营山县"]
      }
    ]
  },
  {
    province:"北京",
    city:[
      {  cityName:"北京市",
        county:["东城区","朝阳区"]
      }
    ]
  },
  {
    province:"安徽",
    city:[
      {  cityName:"安庆",
        county:["安庆市","怀宁县","潜山县"]
      },
      {  cityName:"蚌埠",
        county:["蚌埠市","固镇县","怀远县"]
      }
    ]
  }
]
var opt0 = ["省份","地级市","市、县级市、县"];
var selectID_arr2=["provinceid","cityid","countyid"];
var province_index;
window.onload = function(){
  //初始化下拉框
  function init_select(){
    init_title();
    init_province();
    bind_province();
  }
  //初始化提示标题
  function init_title(){
    for(var k = 0;k<selectID_arr2.length;k++){
      var select_obj= document.getElementById(selectID_arr2[k]);
      select_obj.options[0]=new Option(opt0[k],opt0[k]);
    }
  }
  //初始化省份
  function init_province(){
    var province_select_obj = document.getElementById(selectID_arr2[0]);
    var province_len = province_city_county_data.length;
    for(var i = 0;i<province_len;i++){
      province_select_obj.options[i+1] = new Option(province_city_county_data[i].province,province_city_county_data[i].province);
    }
  }
  //给省份绑定onchange事件
  function bind_province(){
    var province_select_obj = document.getElementById(selectID_arr2[0]);
    province_select_obj.onchange=function(){
      var opt_index =province_select_obj.selectedIndex;
      if(opt_index!=0){
        province_index = opt_index-1;  //每个省份的序号比 option 的下标要小1
        init_city(province_index);
        bind_city();
        init_county(province_index,0);
      }else{
        clear_city();
        clear_county();
      }
    }
  }
  //选择一个省份才能初始化地级市
  function init_city(index){
    clear_city();
    var city_len = province_city_county_data[index].city.length;
    for(var i = 0;i<city_len;i++){
      document.getElementById(selectID_arr2[1]).options[i+1] = new Option(province_city_county_data[index].city[i].cityName,province_city_county_data[index].city[i].cityName);
    }
    document.getElementById(selectID_arr2[1]).options[1].selected = true;
  }
  //清除地级市信息
  function clear_city(){
    var city_select_obj = document.getElementById(selectID_arr2[1]);
    city_select_obj.options.length=0; //每次选中一个新的省份 都重新删除地级市的信息
    init_title();   //重新初始化标题
  }
  //给地级市绑定onchange事件
  function bind_city(){
    var city_select_obj = document.getElementById(selectID_arr2[1]);
    city_select_obj.onchange=function(){
      var opt_index =city_select_obj.selectedIndex;
      if(opt_index!=0){
        init_county(province_index,opt_index-1);
      }else{
        clear_county();
      }
    }
  }
  //选择一个地级市后才能初始化县
  function init_county(index,city_index){
    clear_county();
    var county_len = province_city_county_data[index].city[city_index].county.length;
    for(var i = 0;i<county_len;i++){
      document.getElementById(selectID_arr2[2]).options[i+1] = new Option(province_city_county_data[index].city[city_index].county[i],province_city_county_data[index].city[city_index].county[i]);
    }
    document.getElementById(selectID_arr2[2]).options[1].selected = true;
  }
  //清除县城信息
  function clear_county(){
    var county_select_obj = document.getElementById(selectID_arr2[2]);
    county_select_obj.options.length=0; //每次选中一个新的地级市 都重新删除县的信息
    init_title();   //重新初始化标题
  }
  init_select()
}

<select id="provinceid"></select>
<select id="cityid"></select>
<select id="countyid"></select>