<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Hello World</title>
<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
</head>
<body>
<div class="navbar navbar-default">
<div>
<a href="#">某管理某系统</a>
</div>
<ul class="nav navbar-nav">
<li><a href="#">首页</a></li>
<li>
<a href="#" data-toggle="dropdown">功能<span></span></a>
<ul role="menu" id="dropdownMenu1">
<li>业务功能</li>
<li><a href="#">信息建立</a></li>
<li><a href="#">信息查询</a></li>
<li><a href="#">信息管理</a></li>
<li></li>
<li>系统功能</li>
<li><a href="#">设置</a></li>
</ul>
</li>
<li><a href="#">帮助</a></li>
</ul>
<form class="navbar-form form-inline navbar-right">
<div>
<input type="text" id="exampleInputName2" placeholder="用户名">
</div>
<div>
<input type="email" id="exampleInputEmail2" placeholder="密码">
</div>
<button type="submit" class="btn btn-default">登陆</button>
</form>
</div>
<script src="http://cdn.bootcss.com/jquery/1.11.1/jquery.min.js">
</script>
<script src="http://cdn.bootcss.com/bootstrap/3.2.0/js/bootstrap.min.js">
</script>
</body>
</html>
在ul标签上添加class="dropdown-menu"，就行了