<script type="text/javascript" src="jquery-3.0.0.js"></script>
<script type="text/javascript">
      $(function() {
        //获取所有li遍历
        $(".li_list").each(function() {
          //当鼠标进去当前li
          $(this).mouseenter(function() {
            //设置当前元素宽度
            $(this).stop()
                .animate({ "width": "600px" }, 500, "linear");
                //设置同胞元素宽度
                .siblings().stop()
                      .animate({ "width": "100px" }, 500, "linear");
          });
        });
      });
    </script>

 .li_list {
        width: 100px;
        height: 300px;
        list-style: none;
        float: left;
        overflow: hidden;
      }

      .li_list img {
        width: 100%;
        height: 100%;
      }

      .divbg {
        width: 600px;
        height: 300px;
        background: rgba(230, 0, 0, 0.5);
        text-align: center;
        line-height: 300px;
        float: left;
      }

      .divbg span {
        display: block;
        width: 100px;
        height: 300px;
        float: left;
      }

      .div1 {
        width: 500px;
        height: 300px;
        float: left;
      }
      .mo{
        width: 600px;
      }

<ul class="ul_list">
      <li class="li_list">
        <div class="divbg">
          <span>萌宠</span>
          <div class="div1">
            <img src="img/0.jpg" />
          </div>
        </div>
      </li>
      <li class="li_list">
        <div class="divbg">
          <span>萌宠</span>
          <div class="div1">
            <img src="img/1.jpg" />
          </div>
        </div>
      </li>
      <li class="li_list">
        <div class="divbg">
          <span>萌宠</span>
          <div class="div1">
            <img src="img/2.jpg" />
          </div>
        </div>
      </li>
      <li class="li_list mo">
        <div class="divbg">
          <span>萌宠</span>
          <div class="div1">
            <img src="img/3.jpg" />
          </div>
        </div>
      </li>
    </ul>