.main
{
height: 250px;
background-color: #FFF;
margin-top: 5px;
}
.news
{
width: 340px;
border: 1px solid #E8E8E8;
background-color: #9FC;
}
.course
{
width: 410px;
border: 1px solid #E8E8E8;
background-color: #FF6;
margin: 7px;
}
.sidebar
{
width: 230px;
background-color: #09F;
}
.news,.course,.sidebar
{
height: 250px;
float: left;
}
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<link rel="stylesheet" type="text/css" href="style.css" />
<script src="js/setHomeSetFav.js" type="text/javascript" charset="gb2312"></script>
<script src="js/myfocus-2.0.1.min.js" type="text/javascript"></script>
<script src="js/mf-pattern/mF_YSlider.js" type="text/javascript"></script>
<link href="js/mf-pattern/mF_YSlider.css" rel="stylesheet" type="text/css" />
<script type="text/javascript">
myFocus.set({
id:'picBox'
})
</script>
</head>
<body>
<div class="top">
<div class="top_content">
<ul>
<li><a href="#">联系我们</a></li>
<li><a href="#" onclick="AddFavorite(window.location,Document.title)">加入收藏</a></li>
<li><a href="#" onclick="SetHome(window.location)">设为首页</a></li>
</ul>
</div>
</div><!--top结束-->
<div class="wrap">
<div class="logo">
<div class="logo_left">
<img src="images/logo.jpg" alt="慕课网" />
</div>
<div class="logo_right">
<img src="images/tel.jpg" alt="服务热线" />24小时服务热线:<span class="tel">123-456-7890</span>
</div>
</div><!--logo结束-->
<div class="clear"></div>
<div class="nav">
<div class="nav_left"></div>
<div class="nav_mid">
<div class="nav_mid_left">
<ul>
<li><a href="#">首页</a></li>
<li><a href="list.html">关于慕课</a></li>
<li><a href="list.html">新闻动态</a></li>
<li><a href="list.html">课程中心</a></li>
<li><a href="list.html">在线课程</a></li>
<li><a href="list.html">人才招聘</a></li>
</ul>
</div>
<div class="nav_mid_right">
<form action="" method="post">
<input type="text" class="search_text" />
</form>
</div>
</div><!--nav_mid结束-->
<div class="nav_right"></div>
</div><!--nav结束-->
<div class="ad" id="picBox">
<div class="loading"><img src="images/loading.gif" alt="图片加载中" /></div>
<div class="pic"> <!-- 图片列表外面，必须包裹一个class为pic的div-->
<ul>
<li><img src="images/ad2.jpg" /></li>
<li><img src="images/ad3.jpg" /></li>
<li><img src="images/ad4.jpg" /></li>
</ul>
</div>
</div><!--ad结束-->
<div class="main">
<div class="news">
<div class="title"></div>
<div class=""></div>
<div class=""></div>
</div><!--news结束-->
<div class="course"></div><!--course结束-->
<div class="sidebar"></div><!--sidebar结束-->
</div><!--main结束-->
</div><!--wrap结束-->
</body>
</html>
course的margin值写错了
应该是margin:0 7px; 你没写上下的间距设为0 直接写7px默认为上下左右均为7px！
三个盒子的上下位置是不是不一样