cities = new Object();
cities[‘河北省‘]=new Array(‘石家庄‘, ‘张家口市‘, ‘承德市‘, ‘秦皇岛市‘, ‘唐山市‘, ‘廊坊市‘, ‘保定市‘, ‘沧州市‘, ‘衡水市‘, ‘邢台市‘, ‘邯郸市‘);
cities[‘山西省‘]=new Array(‘太原市‘, ‘大同市‘, ‘朔州市‘, ‘阳泉市‘, ‘长治市‘, ‘晋城市‘, ‘忻州地区‘, ‘吕梁地区‘, ‘晋中市‘, ‘临汾地区‘, ‘运城地区‘);

function set_city(province, city)
{
    var pv, cv;
    var i, ii;

    pv=province.value;
    cv=city.value;

    city.length=1;

    if(pv==‘0‘) return;
    if(typeof(cities[pv])==‘undefined‘) return;

    for(i=0; i<cities[pv].length; i++)
    {
       ii = i+1;
       city.options[ii] = new Option();
       city.options[ii].text = cities[pv][i];
       city.options[ii].value = cities[pv][i];
    }

}

 <form name="form1" action="" method="get">
    <select name="province" onChange="set_city(this, this.form.city);">
    <option value=http://www.mamicode.com/"0">选择省</option>
    <option value=http://www.mamicode.com/"河北省">河北省</option>
    <option value=http://www.mamicode.com/"山西省">山西省</option>
    </select>
    <select   name="city" id="citys">
    <option value=http://www.mamicode.com/"0">选择城市</option>
    </select>
   </form>