.header{
width: 100%;
height: 555rpx;
background:linear-gradient(to top, rgb(255, 255, 255,.8) 10%,transparent),url('https://wx-img-gpdk.oss-cn-beijing.aliyuncs.com/new/WechatIMG2295.png') ;
background: -o-linear-gradient(to top, rgb(255, 255, 255,.8) 10%,transparent),url('https://wx-img-gpdk.oss-cn-beijing.aliyuncs.com/new/WechatIMG2295.png');
background: -moz-linear-gradient(to top, rgb(255, 255, 255,.8) 10%,transparent),url('https://wx-img-gpdk.oss-cn-beijing.aliyuncs.com/new/WechatIMG2295.png');
background: -webkit-linear-gradient(to top, rgb(255, 255, 255,.8) 10%,transparent),url('https://wx-img-gpdk.oss-cn-beijing.aliyuncs.com/new/WechatIMG2295.png');
background-size: cover;
background-repeat: no-repeat;
position: relative;
}
你那是rgba 不是rgb吧
https://developers.weixin.qq.com/s/LTzdtYmW7Yh9