代码如下：

<!DOCTYPE html>
<html>
<head>
<script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
<meta charset="utf-8">
<title></title>
<style type="text/css">
ul ul {display: none;}
</style>
</head>
<body>
<ul class="nav">
<li>
<span>一级菜单</span>
<ul>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
</ul>
</li>
<li>
<span>一级菜单</span>
<ul>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
</ul>
</li>
<li>
<span>一级菜单</span>
<ul>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
<li>二级菜单</li>
</ul>
</li>
</ul>
</body>
<script src="jquery.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript">
$(".nav").children("li").mouseover(function(){
$(this).children("ul").stop().show(500)
.parent().siblings().children("ul").stop().hide(500);
})
</script>
</html>

效果如图：

jquery实现二级菜单_二级菜单