<li>
   <h3><i class="fa fa-fw fa-cog" ></i>系统设置</h3>
    <ul class="sub_menu" style="display: block">
        <li><a href="http://www.mamicode.com/#" target="main"><i class="fa fa-fw fa-cubes"></i>网站配置</a></li>
        <li><a href="http://www.mamicode.com/#" target="main"><i class="fa fa-fw fa-database"></i>备份还原</a></li>
    </ul>
</li>

关键代码就是

style="display: block"

实习日记 怎么让菜单栏默认打开