方法1:宽度和高度已知的。 思路： 给父元素相对定位 给子元素绝对定位 left: 50%;top: 50%; margin-left: 负的宽度一半。 margin-top: 负的高度一半；
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>居中</title> <style type="text/css"> #box{ width: 400px; height: 200px; position: relative; background: red; } #box1{ width: 200px; height: 100px; position: absolute; top: 50%; left: 50%; margin-left: -100px; margin-top: -50px; background: green; } </style> </head> <body> <div id="box"> <div id="box1"> </div> </div> </body> </html>方法2：宽度和高度自己未知 意思就是说子盒子本身还是有宽度和高度的，只是自己未知。 思路： 给父盒子相对定位 给子盒子绝对定位 top、right、bottom、left全为0 margin: auto;
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>居中</title> <style type="text/css"> #box{ width: 800px; height: 400px; position: relative; background: red; } #box1{ width: 100px; height: 50px; position: absolute; top: 0; right: 0; bottom: 0; left: 0; margin: auto; background: green; } </style> </head> <body> <div id="box"> <div id="box1"> </div> </div> <script type="text/javascript"> </script> </body> </html>方法3：flex布局
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>垂直居中</title> <style type="text/css"> .box{ width: 400px; height: 200px; background: #f99; } .box1{ width: 200px; height: 100px; background: green; } .center{ display: flex; justify-content: center; align-items: center; } </style> </head> <body> <div class="box center"> <div class="box1"> </div> </div> </body> </html>方法四：平移
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>css3让一个盒子居中</title> <style type="text/css"> .parent_box{ width: 400px; height: 200px; background: red; position: relative; } .child_box{ width: 200px; height: 100px; background: #9ff; position: absolute; top: 50%; left: 50%; transform: translate( -50%,-50%); } </style> </head> <body> <div class="parent_box"> <div class="child_box"> </div> </div> </body> </html>