options不用一个一个去删除,把它长度设置为0就可以了
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
    <title>无标题文档</title>
<script type="text/javascript">
var Provincesname={
name:"湖北省,湖南省"
}
var cityname={
"湖北省":"黄冈,武汉,黄梅,宜昌,武穴,黄石,英山,仙桃,天门,孝感,荆州,荆门,鄂州,恩施",
"湖南省":"长沙,株洲,大海"
}
function loadata(){
var Provinces=document.getElementById("Provinces");
var Provincesarray=Provincesname.name.split(",");
with(Provinces){
for(var i=0;i<Provincesarray.length;i++){
 add(new Option(Provincesarray[i]));
}
}
}
function selectcity(obj){
var city=document.getElementById("city");
city.options.length = 0;
var provincesname=obj.options[obj.selectedIndex].text;
var citys=cityname[provincesname];
var cityarray=citys.split(",");
with(city){
for(var i=0;i<cityarray.length;i++){
 add(new Option(cityarray[i]));
}
}
}
</script>
</head>
<body onload="loadata()">
<select id="Provinces" size="1" onchange="selectcity(this)"  style="width:100px;">
  <option selected="selected">请选中省份</option>
</select>
<select id="city" size="1" style="width:100px;">
    <option selected="selected">请选中城市</option>
</select>
</body>
</html>