<volist name="list" id="vo">

    {$vo.prod_cate_name}

    <volist name="vo['voo']" id="sub">

        {$sub.prod_cate_name}

    </volist>

</volist>

$Dao = M('ProductCategory');

$list = $Dao->where('ifnull(prod_cate_pid,0)=0')->order('prod_cate_order asc')->findAll();

$Sub = M('ProductCategory');

foreach ($list as $n=>$val)

{

    $list[$n]['voo']=$Sub->where('prod_cate_pid='.$val['prod_cate_id'])->select();

}

$this->assign('list',$list);

$this->display();

<script type="text/javascript" src="__ROOT__/public/scripts/jquery-1.7.1.min.js"></script> 

<script type="text/javascript">

function ajaxSelect(url,objone,objtwo,defval){

    var $s1=$(objone);

    var $s2=$(objtwo);

    $s1.change(function(){

        var curval = objone.options[objone.selectedIndex].value;

        $s2.html("");

        $("<option>").text("二级分类").val("").appendTo($s2);

        $.ajax({ 

             type: "POST", 

             url: url, 

             data: "catepid="+curval,

             dataType: 'json',

             success: function(obj){

                if(obj.data)

                {

                    $(eval(obj.data)).each(function(i,elm){

                        appendOptionTo($s2,elm.prod_cate_name,elm.prod_cate_id,defval);

                    });

                }

             }

        });

    }).change();

    function appendOptionTo(o,k,v,d){

        var opt=$("<option>").text(k).val(v);

        if(v==d){opt.attr("selected", "selected")}

        opt.appendTo(o);

    }

}

$(function(){

    ajaxSelect('__URL__/changeCategory',$('#cate_one').get(0),$('#cate_two').get(0),'{$vo.cate_two}');

</script>

<tagLib name="html" />

<html:select options="productCategory" selected="prodCateId" first="选择分类" name="cate_one" id="cate_one" /> 

<select name="cate_two" id="cate_two">

<option value="">二级分类</option>

</select>

public function changeCategory()

{

    $id= $_POST['catepid'];

    header("Content-type:text/html; charset=utf-8");

    $Cate = D('ProductCategory');

    $list = $Cate->field('prod_cate_id,prod_cate_name')->where('prod_cate_pid='.$id)->select();

    $this->ajaxReturn(json_encode($list),'','',0);

}