jQuery 下拉菜单添加动画demo效果：

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
        margin: 0;
        padding: 0;
    }

    ul {
        list-style: none;
    }

    .wrap {
        width: 330px;
        height: 30px;
        margin: 100px auto 0;
        padding-left: 10px;
        background-color: yellow;
    }

    .wrap li{
        background-color: red;
    }

    .wrap > ul > li {
        float: left;
        margin-right: 10px;
        position: relative;
    }

    .wrap a {
        display: block;
        height: 30px;
        width: 100px;
        text-decoration: none;
        color: #000;
        line-height: 30px;
        text-align: center;
    }

    .wrap li ul {
        position: absolute;
        top: 30px;
        display: none;
    }
  </style>
  <script src="jquery-3.4.1.js"></script>
  <script>
    // 入口函数
    $(document).ready(function () {
      // 鼠标放入一级Li中，让他里面的ul显示，一开隐藏
      var jqLi = $('.wrap>ul>li');

      // 绑定事件(显示)
      jqLi.mouseenter(function () {
        $(this).children('ul').slideDown()
      })

      // 绑定事件(隐藏)
      jqLi.mouseleave(function () {
        $(this).children('ul').slideUp()
      })
    })
  </script>
</head>
<body>
  <div class="wrap">
    <ul>
        <li>
            <a href="javascript:void(0);">一级菜单1</a>
            <ul>
                <li><a href="javascript:void(0);">二级菜单1</a></li>
                <li><a href="javascript:void(0);">二级菜单2</a></li>
                <li><a href="javascript:void(0);">二级菜单3</a></li>
            </ul>
        </li>
        <li>
            <a href="javascript:void(0);">一级菜单1</a>
            <ul>
                <li><a href="javascript:void(0);">二级菜单1</a></li>
                <li><a href="javascript:void(0);">二级菜单2</a></li>
                <li><a href="javascript:void(0);">二级菜单3</a></li>
            </ul>
        </li>
        <li>
            <a href="javascript:void(0);">一级菜单1</a>
            <ul>
                <li><a href="javascript:void(0);">二级菜单1</a></li>
                <li><a href="javascript:void(0);">二级菜单2</a></li>
                <li><a href="javascript:void(0);">二级菜单3</a></li>
            </ul>
        </li>
    </ul>
  </div>
</body>
</html>
demo 效果：
jQuery 下拉菜单 添加动画

jQuery 下拉菜单添加动画
