苹果手机里面背景图片的fixed属性会失效，使用:before元素解决
body{
width: 100%;
height:auto;
background:url("../img/bg.jpg") no-repeat fixed;
background-size: cover;
}
body:before {
content: ' ';
position: fixed;
z-index: -1;
top: 0;
right: 0;
bottom: 0;
left: 0;
background: url(../img/bg.jpg) center 0 no-repeat;
background-size: cover;
}
