<div><ul><li><a>1</a></li><li><a>2</a></li></ul></div>，然后调好样式。并将div的style设置，display=none
然后获取到你的栏目，添加鼠标事件onMouseover或onclick都行。事件中添加jquery代码。
$("要下来的那个div的选择器").slideDown(100) 100为时间
基本上就是这个思路。
这个是用JS实现的。你百度一下，JS实现二级导航源码就有了。
<!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>简洁的纯CSS下拉菜单代码</title>
<style>
#sddm
{ margin: 0 auto
padding: 0
z-index: 30
background-color:#F4F4F4
width: 480px
height:23px}
#sddm li
{ margin: 0
padding: 0
list-style: none
float: left
font: bold 12px arial}
#sddm li a
{ display: block
margin: 0 1px 0 0
padding: 4px 10px
width: 60px
background: #5970B2
color: #FFF
text-align: center
text-decoration: none}
#sddm li a:hover
{ background: #49A3FF}
#sddm div
{ position: absolute
visibility: hidden
margin: 0
padding: 0
background: #EAEBD8
border: 1px solid #5970B2}
#sddm div a
{ position: relative
display: block
margin: 0
padding: 5px 10px
width: auto
white-space: nowrap
text-align: left
text-decoration: none
background: #EAEBD8
color: #2875DE
font: 12px arial}
#sddm div a:hover
{ background: #49A3FF
color: #FFF}
</style>
<!-- dd menu -->
<script type="text/javascript">
<!--
var timeout = 100
var closetimer = 10
var ddmenuitem = 0
// open hidden layer
function mopen(id)
{
// cancel close timer
mcancelclosetime()
// close old layer
if(ddmenuitem) ddmenuitem.style.visibility = 'hidden'
// get new layer and show it
ddmenuitem = document.getElementById(id)
ddmenuitem.style.visibility = 'visible'
}
// close showed layer
function mclose()
{
if(ddmenuitem) ddmenuitem.style.visibility = 'hidden'
}
// go close timer
function mclosetime()
{
closetimer = window.setTimeout(mclose, timeout)
}
// cancel close timer
function mcancelclosetime()
{
if(closetimer)
{
window.clearTimeout(closetimer)
closetimer = null
}
}
// close layer when click-out
document.onclick = mclose
// -->
</script>
</head>
<body style="text-align:center">
<ul id="sddm">
<li><a href="#" onmouseover="mopen('m1')" onmouseout="mclosetime()">Home</a>
<div id="m1" onmouseover="mcancelclosetime()" onmouseout="mclosetime()">
<a href="#">HTML DropDown</a>
<a href="#">DHTML DropDown menu</a>
<a href="#">JavaScript DropDown</a>
<a href="#">DropDown Menu</a>
<a href="#">CSS DropDown</a>
</div>
</li>
<li><a href="#" onmouseover="mopen('m2')" onmouseout="mclosetime()">Download</a>
<div id="m2" onmouseover="mcancelclosetime()" onmouseout="mclosetime()">
<a href="#">ASP Dropdown</a>
<a href="#">Pulldown menu</a>
<a href="#">AJAX dropdown</a>
<a href="#">DIV dropdown</a>
</div>
</li>
<li><a href="#" onmouseover="mopen('m3')" onmouseout="mclosetime()">Order</a>
<div id="m3" onmouseover="mcancelclosetime()" onmouseout="mclosetime()">
<a href="#">Visa Credit Card</a>
<a href="#">Paypal</a>
</div>
</li>
<li><a href="#" onmouseover="mopen('m4')" onmouseout="mclosetime()">Help</a>
<div id="m4" onmouseover="mcancelclosetime()" onmouseout="mclosetime()">
<a href="#">Download Help File</a>
<a href="#">Read online</a>
</div>
</li>
<li><a href="#" onmouseover="mopen('m5')" onmouseout="mclosetime()">Contact</a>
<div id="m5" onmouseover="mcancelclosetime()" onmouseout="mclosetime()">
<a href="#">E-mail</a>
<a href="#">Submit Request Form</a>
<a href="http://www.codefans.net/" target="_blank">codefans.net</a>
</div>
</li>
</ul>
</body>
</html>
检查一下poaition和overflow的值设置是否合理。可以调出F12开发人员工具（右键点击对应的元素，然后选择“审查元素”或“检查元素”，看看那些css有什么设置，还可以现场临时修改以尝试正确的css）
