Jquery消除动画闪现效果之后的异常_问答-阿里云开发者社区

这是我的代码：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
  <head>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
    <script type="text/javascript" src="js/jquery.js"></script>
<script type="text/javascript" src="js/menu.js"></script>
<link rel="stylesheet" type="text/css" href="css/menu.css" />
  </head>
  
  <body>
     <div id="menu_body_id">
      <ul>
      <li class="parent_menu_li_class">公司介绍
      
      <ul class="son_menu_ul_class">
      <li class="son_menu_li_class">历史沿岸</li>
      <li class="son_menu_li_class">高管层</li>
      <li class="son_menu_li_class">公司荣誉</li>
      </ul>
      
      </li> 
      <li class="parent_menu_li_class">新闻资讯
      
      <ul class="son_menu_ul_class">
      <li class="son_menu_li_class">行业新闻</li>
      <li class="son_menu_li_class">国内新闻</li>
      <li class="son_menu_li_class">世界新闻</li>
      </ul>
      
      </li>
      <li class="parent_menu_li_class">社会责任</li>
      </ul>
     </div>
  </body>
</html>

css如下：

@CHARSET "UTF-8";
* {
padding:0; 
margin:0;
}
body,div {
padding:0; 
margin:0;
}
body {
width:160px;
height:50px;
}
th td {
padding:0; 
margin:0;
}
#menu_body_id {

}
#menu_body_id ul li{
list-style-type:none;
font-family: "宋体"; 
font-size: 18px; 
line-height: 20px;
background-color:#9999FF;
text-align:left;
width:100%;
height:24px;
padding:1 1 1 0px; 
margin:2px;
list-style: none;
float:left;
postion:relative;
display:inline;
}
#menu_body_id ul li ul li {
list-style-type:none;
font-family: "宋体"; 
font-size: 18px; 
line-height: 20px;
background-color:#99FFFF;
text-align:left;
width:100%;
height:24px;
padding:1 1 1 0px; 
margin:1 0 0 0px;
list-style: none;
float:left;
postion:relative;
display:inline;
}


.parent_menu_li_class {
}
.son_menu_ul_class {
}
.son_menu_li_class {
}
js如下：

$(document).ready(function () {
  $(".son_menu_ul_class").hide();
$(".parent_menu_li_class").click(function () {
//$("#gsjs_zzjg_dalian_id").siblings("#gsjs_zzjg_menu_id").fadeIn();
$(this).find(".son_menu_ul_class").slideDown("slow"); 
});
});

但是效果却是这样的：