<script language = "JavaScript">
var onecount;
onecount=0;
subcat = new Array();
        
subcat[0] = new Array("操作系统","1","1");
        
subcat[1] = new Array("网络安全","1","2");
        
subcat[2] = new Array("软件破解","1","3");
        
subcat[3] = new Array("心情日记","2","4");
        
subcat[4] = new Array("笑话幽默","3","5");
        
subcat[5] = new Array("灌水用帖","3","6");
        
onecount=6;

function changelocation(locationid)
    {
    document.myform.Nclassid.length = 0; 

    var locationid=locationid;
    var i;
    for (i=0;i < onecount; i++)
        {
            if (subcat[i][1] == locationid)
            { 
                document.myform.Nclassid.options[document.myform.Nclassid.length] = new Option(subcat[i][0], subcat[i][2]);
            }        
        }
        
    }    
</script>


<form method="POST" name="myform"">

  <select name="classid" onChange="changelocation(document.myform.classid.options[document.myform.classid.selectedIndex].value)" size="1">
    <option class=input selected value="1">电脑相关</option>
    <option class=input value="2">情感频道</option>
    <option class=input value="3">休闲娱乐</option>
  </select> 
    
  <select name="Nclassid" class=input>                  
    <option class=input selected value="1">操作系统</option>
    <option class=input value="2">网络安全</option>
    <option class=input value="3">软件破解</option>
  </select>

</form>

------解决方案--------------------
太多了
http://www.google.com.hk/search?client=aff-cs-maxthon&forid=1&ie=utf-8&oe=UTF-8&hl=zh-CN&q=%E7%BA%A7%E8%81%94%E8%8F%9C%E5%8D%95

------解决方案--------------------
用JQUERY
 做級聯更方便 更容易。。。

------解决方案--------------------
JS啊,很简单的。。

------解决方案--------------------
................................

------解决方案--------------------
用JS很容易就实现了

------解决方案--------------------
    最好就是用Ajax，这是它的特性之一。

------解决方案--------------------
先找个例子看下，一会就弄明白了

------解决方案--------------------
用js就好了~~~
 用js的一个类似于map的集合做
很好做的