tag = document.getElementById('span_big_class');
tag.innerHTML = "<select class=\'big_class\' name=big_class_id onchange=\'show_small_class(this.options[selectedIndex].value)\'><option value=\'\'>产品大类</option><option value=\'3\' >手机</option><option value=\'7\' >本本整机</option><option value=\'1\' >数码产品</option><option value=\'8\' >DIY配件</option><option value=\'2\' >数码配件</option><option value=\'14\' >数字家电</option><option value=\'9\' >网络设备</option><option value=\'12\' >扩展配件</option><option value=\'11\' >外设办公</option><option value=\'13\' >耗材相关</option><option value=\'16\' >小家电</option><option value=\'17\' >厨卫家电</option><option value=\'15\' >软件产品</option></select> ";

function show_small_class(big_class_id)
{
    tag = document.getElementById('script_small_class');
    tag.src = "http://yp.pchome.net/class/show_small_class.php?big_class_id="+big_class_id;
}