body {
  background: url('https://www.blender.org/wp-content/uploads/2013/04/gleb-alexandrov-sunrise-02.jpg') center top;  
  background-attachment: fixed;  
                /*位置固定*/  
}
#main {  
         
                overflow: hidden;  
              
                box-shadow: 0 15px 20px rgba(0, 0, 0, 0.5);  
                z-index: 100;  
                padding: 50px;  
                box-sizing: border-box;  
                /*不会把盒子撑开*/  
               position:relative;
              
            }  

            #main::before {  
                content: "";  
                position: absolute;  
                top: 0px;  
                left: 0px;  
                right: 0px;  
                bottom: 0px;  
                z-index: -1;  
                /*-1 可以当背景*/  
                -webkit-filter: blur(10px);  
                filter: blur(10px);  
                margin: -30px;  /*消除边缘透明*/  
                background: url('https://www.blender.org/wp-content/uploads/2013/04/gleb-alexandrov-sunrise-02.jpg') center top;  
                background-size: cover;  
                /*平铺*/  
                background-attachment: fixed;  
                /*位置固定*/  
            }