父级元素overflow-y: scroll; 里面的子元素绝对定位被隐藏。

<style>         *{             margin: 0;             padding: 0;         }         .side{             float: right;             width: 300px;             height: 100%;             overflow-y: auto;         }         .item{             position: relative;             margin: 10px 0;         }         .item-hd{             background-color: #ccc;             font-size: 14px;             line-height: 30px;             text-align: center;         }         .item-bd{             display: none;             position: absolute;             top:0;             right: 280px;             width: 300px;             height: 200px;             background-color: #f00;             z-index: 9;         }         .item:hover .item-bd{             display: block;         }     </style> <div class="side">         <div style="height:300px;"></div>         <div class="box">             <div class="hd">XXXX</div>             <div class="bd">                 <div class="item">                     <div class="item-hd">2333</div>                     <div class="item-bd">                         666                     </div>                 </div>                 <div class="item">                     <div class="item-hd">2333</div>                     <div class="item-bd">                         666                     </div>                 </div>                 <div class="item">                     <div class="item-hd">2333</div>                     <div class="item-bd">                         666                     </div>                 </div>                 <div class="item">                     <div class="item-hd">2333</div>                     <div class="item-bd">                         666                     </div>                 </div>                 <div class="item">                     <div class="item-hd">2333</div>                     <div class="item-bd">                         666                     </div>                 </div>                 <div class="item">                     <div class="item-hd">2333</div>                     <div class="item-bd">                         666                     </div>                 </div>             </div>         </div>          <div style="height:800px;"></div>     </div>

在线代码地址：http://runjs.cn/code/x3l82ejf