假如 box 的高和宽为100px

 .centered_div { width: 100px; height: 100px; position: absolute; top: 50%; left: 50%; margin-left: -50px; margin-top: -50px; background: red; } 

假如 box的宽度为500px 高度为200px

  .centered_div { width:500px; height: 200px; position: absolute; top: 50%; left: 50%; margin-left: -250px; margin-top: -100px; background: red; }