html

<div id="bj"></div>

css

#bj {
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: url(https://xn--xu0a.cn/api/dn/) center center no-repeat;
    background-size: cover;
    position: fixed;
    transition: all 0.3s;
    background-size: cover;
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: 50% 50%;
    z-index: -1;
}