解决width: 100%;再设置padding问题

在后面加上box-sizing: border-box;就OK了

.footer_nav{
    width: 100%;
    overflow:hidden;
    margin-bottom:0;
    position:fixed;
    bottom:0;
    background: linear-gradient(to right, rgb(255, 103, 109), rgb(255, 141, 111));
    z-index:3;
    left: 0;
    padding: 8px;
    box-sizing: border-box;
}