二级联动，例：根据省份选择市区

<script type="text/javascript">
        var array = [["--请选择--"],["海淀区","朝阳区","石景山区","昌平区"],["青岛市","济南市","泰安市","聊城市"],["郑州市","南阳市","安阳市","驻马店"],["石家庄","廊坊","沧州","邯郸"],["西安","咸阳","大同"]];
        
        function sel(select){
            
            var optionIndex = select.selectedIndex;
            var options = array[optionIndex];
                       
            //获取二级select对象
            var selectTwo = document.getElementById("two");
            
            //把当前select下的option清空
            selectTwo.options.length=0;

            for (var index in options) {
                var optionObj = document.createElement("option");
                optionObj.innerHTML=options[index];
                selectTwo.appendChild(optionObj);
            }
                      
        }
               
    </script>

 1 <select onchange="sel(this)">
 2             <option>--请选择--</option>
 3             <option>北京市</option>
 4             <option>山东省</option>
 5             <option>河南省</option>
 6             <option>河北省</option>
 7             <option>山西省</option>
 8         </select>
 9         
10         
11         <select id="two">
12                    
13         </select>