效果如下：

第一步,插入相關的js和css檔案

<link rel="stylesheet" href="http://libs.baidu.com/bootstrap/3.0.3/css/bootstrap.min.css"/>

<script type="text/javascript" src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script>
<script type="text/javascript" src="http://libs.baidu.com/bootstrap/3.0.3/js/bootstrap.min.js"
 
></script>

第二步，單獨的一個css樣式

.dropdown-submenu {
            position: relative;
        }
        .dropdown-submenu > .dropdown-menu {
            top: 0;
            left: 100%;
            margin-top: -6px;
            margin-left: -1px;
            -webkit-border-radius: 0 6px 6px 6px;
            -moz-border-radius: 0 6px 6px;
            border-radius: 0 6px 6px 6px;
        }
        .dropdown-submenu:hover > .dropdown-menu {
            display: block;
        }
        .dropdown-submenu > a:after {
            display: block;
            content: " ";
            float: right;
            width: 0;
            height: 0;
            border-color: transparent;
            border-style: solid;
            border-width: 5px 0 5px 5px;
            border-left-color: #ccc;
            margin-top: 5px;
            margin-right: -10px;
        }
        .dropdown-submenu:hover > a:after {
            border-left-color: #fff;
        }
        .dropdown-submenu.pull-left {
            float: none;
        }
        .dropdown-submenu.pull-left > .dropdown-menu {
            left: -100%;
            margin-left: 10px;
            -webkit-border-radius: 6px 0 6px 6px;
            -moz-border-radius: 6px 0 6px 6px;
            border-radius: 6px 0 6px 6px;
        }
 

第三步，HTML程式碼

<div class="container">
    <div class="row">
        <h2>Bootstrap 3多級下拉選單</h2>
        <hr>
        <div class="dropdown">
            <a id="dLabel" role="button" data-toggle="dropdown" class="btn btn-primary" data-target="#"
               href="javascript:;">
                下拉多級選單 <span class="caret"></span>
            </a>
            <ul class="dropdown-menu multi-level" role="menu" aria-labelledby="dropdownMenu">
                <li><a href="javascript:;">一級選單</a></li>
                <li><a href="javascript:;">一級選單</a></li>
                <li class="divider"></li>
                <li class="dropdown-submenu">
                    <a tabindex="-1" href="javascript:;">一級選單</a>
                    <ul class="dropdown-menu">
                        <li><a tabindex="-1" href="javascript:;">二級選單</a></li>
                            <li class="divider"></li>
                            <li class="dropdown-submenu">
                                <a href="javascript:;">二級選單</a>
                                <ul class="dropdown-menu">
                                    <li><a href="javascript:;">三級選單</a></li>
                                </ul>
                            </li>
                    </ul>
                </li>
            </ul>
        </div>
    </div>
</div>