您需要稍微更改jQuery脚本以适应这样的多级菜单:

You need to change the jQuery script a little to accommodate for the multilevel menus like this:

$('ul#menu li').hover(function(){
     $(this).children('ul').delay(20).slideDown(200);
}, function(){
     $(this).children('ul').delay(20).slideUp(200);
});

并像这样更改html:

and change html like this:

<ul id="menu">
    <li><a href="#">Home</a>
        <ul class="drop">
            <li><a href="#">About us</a></li>
            <li><a href="#">About us</a></li>
            <li><a href="#">About us</a></li>
            <li>
                <a href="#">About us</a>
                <ul>
                    <li><a href="#">Sub Item 1</a></li>
                    <li>
                        <a href="#">Sub Item 2</a>
                        <ul>
                            <li><a href="#">Sub item 3</a></li>
                            <li><a href="#">Sub item 4</a></li>
                        </ul>
                    </li>
                </ul>
            </li>
        </ul>
    </li>


    <li><a href="#">about</a>
        <ul class="drop">
            <li><a href="#">Home</a></li>
            <li><a href="#">Home</a></li>
            <li><a href="#">Home</a></li>
            <li><a href="#">Home</a></li>
        </ul>
    </li>
</ul>

您的CSS没问题. 您可以在以下位置查看更新的多级代码: http://jsfiddle.net/297t6/

Your css is fine. You can check the updated multilevel code at: http://jsfiddle.net/297t6/