<select id="a1" onchange="addOption()"></select>

<select id="a2"></select>

var city = new Array;        

city['江苏'] = ['南京', '连云港', '苏州', '镇江'];        

city['山东'] = ['青岛', '烟台', '济南'];         

function allCity() {            

  var select1 = document.getElementById("a1");            

  for(var i in city) {  //这里注意遍历数组的写法

    select1.add(new Option(i, i), null);

  }   

  addOption(); // 初始化选项     

}        

function addOption() {            

  var select2 = document.getElementById("a2");

  var select1 = document.getElementById("a1").value; 

  select2.length = 0; //每次都先清空一下市级菜单  

  if(select1 != '请选择省份') {                

    for(var i in city[select1 ]) {    

      select2.add(new Option(city[select1][i], city[select1][i]), null);                

    }            

  }else if (sheng == '请选择省份'){              

    select2.length = 0;           

    select2.add(new Option("请选择城市", "请选择城市"), null); 

  }      

}       

window.onload = allCity();