.nav{
float: left;
}
.nav ul li {
float: left;
position: relative;
}
.nav ul li a{
color: #FFF;
font-size: 16px;
line-height: 80px;
margin: 0 25px;
}
.nav ul li:hover:before{
left: 0;
width: 100%;
}
.nav ul li:before {
content: "";
width: 0;
border-bottom: 2px solid #e5081c;
position: absolute;
bottom: 0;
left: 50%;
transition: all linear 0.3s;
}
.nav ul li:hover a{
color: #e5081c;
}
.nav ul .active a{
color: #e5081c;
}
.nav ul .active:before{
left: 0;
width: 100%;
}

			<div class="nav">
				<ul>
					<li class="active"><a href="#">首页</a></li>
					<li><a href="#">关于我们</a></li>
					<li><a href="#">产品中心</a></li>
					<li><a href="#">新闻资讯</a></li>
					<li><a href="#">联系我们</a></li>
				</ul>
			</div>