<form method="post" action="">

         省/市：<select id="province" onchange="alter(this.selectedIndex)"></select>

         市/区：<select id="city"></select>

     </form>

     <script type="text/javascript">

         //定义省/直辖市数组

         var arr_province = ["请选择省/直辖市","北京市","上海市","广东省"];

         //定义市/区二维数组

         var arr_city = [

                         ["请选择市/区"],

                         ["东城区","西城区","朝阳区","宣武区","昌平区","大兴区","丰台区","海淀区"],

                         ['宝山区','长宁区','丰贤区', '虹口区','黄浦区','青浦区','南汇区','徐汇区','卢湾区'],

                         ['广州市','惠州市','汕头市','珠海市','佛山市','中山市','东莞市']

                     ];

         //获取对象

         var province = document.getElementById('province');

         var city = document.getElementById('city');

         //初始化菜单

         onload = function () {

             //指定省option标记的个数

             province.length = arr_province.length;

             //数组数据写入<option>标记中

             for(var i = 0; i < arr_province.length; i++){

                 province.options[i].text = province.options[i].value = arr_province[i];

             }

             //设置省列表默认选项

             var index = 0;

             province.index = index;

             //指定城市option标记的个数

             city.length = arr_city[index].length;

             //数组数据写入option标记

             for (var j = 0; j < arr_city[index].length; j++) {

                 city.options[j].text = city.options[j].value = arr_city[index][j];

             }

         }

         function alter(index) {

             //修改省列表的选择项

             province.index = index;

             //指定城市option标记的个数

             city.length = arr_city[index].length;

             //数组中的数据写入option标记

             for (var j = 0; j < arr_city[index].length; j++) {

                 city.options[j].text = city.options[j].value = arr_city[index][j];

             }

         }

     </script>

　　运行：