SOURCE

<ul>
    <li>
        <a href="#" class="">手机</a>
    </li>
     <li>
        <a href="#" class="">智能硬件</a>
    </li>
</ul>

ul {
    display: flex;
    list-style: none;
}
li {
    height: 60px;
    line-height: 60px;
    margin: 0 5px;
}
a {
    display: inline-block;
    color: black;
    text-decoration: none;
    position: relative;
    transition: all .36s cubic-bezier(0.4,0.8,0.74,1);
    height: 100%;
}
a:hover:after  {
    width: 100%;
}

a:after {
    content: '';
    width: 0;
    display: block;
    position: absolute;
    bottom: 0;
    left: 50%;
    transform: translateX(-50%);
    height: 2px;
    background-color: #000;
    transition: all .36s cubic-bezier(0.4,0.8,0.74,1);
}