这是因为对html整体使用 html{font-size:625%;} 导致行高将p撑出了高度。有人在https://segmentfault.com/q/10...给出了更详细的解答。

css - form表单在没有设置高度和边框内外边距的情况下，过高的问题-Soal Jawab Rangkaian PHP Cina

<!DOCTYPE html>
<html lang="zh-cn">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0,minimum-scale=1.0, maximum-scale=1.0,user-scalable=no">
    <title>移动端</title>
    <style>
    html{font-size:625%;}
        * {
    margin: 0;
    padding: 0;
}
body {
    background-color: #fff;
    font-family: "Helvetica Neue",Helvetica,Arial,"Microsoft Yahei UI","Microsoft YaHei",SimHei,"宋体",simsun,sans-serif;
}
ul,ol {
    list-style: outside none none;
}
a {
    text-decoration: none;
}
img{display:block; width:100%;}
.none {
    display: none;
}
#header{width:100%; height:0.45rem; background:#333;}
#header li{float:left; font-size:0.16rem; line-height:0.45rem; width:25%; text-align:center;}
#header li a{color:#eee;}
#header li:hover{background:#000;}
.footer{max-width:6.4rem; background:#222; margin:0 auto; font-size:0.16rem; text-align:center; color:#777; padding:0.1rem 0;}
.img{max-width:6.4rem; margin:0 auto;}
.sou{max-width:6.4rem; margin:0 auto; background:#ddd; }

    </style>
</head>
<body>
    <p id="header">
        <ul>
            <li><a href="index.html">首页</a></li>
            <li><a href="information.html">资讯</a></li>
            <li><a href="ticket.html">票务</a></li>
            <li><a href="about.html">关于</a></li>
        </ul>
    </p>

    <p class="img">
        <img src="adver.png">
    </p>

    <p class="sou">
        <form action="#" method="post">
            <input type="text" name="text" placeholder="请输入旅游景点或城市">
            <input type="button" value="搜索">
        </form>
    </p>

    <p class="footer">
        <p>客户端 | 触屏版 | 电脑版</p>
        <p>Copyright © YCKU 瓢城旅行社 | 苏ICP备120110119号</p>
    </p>

</body>
</html>