要点：
1.鼠标点击拖动时，获取点击时候的位置，距离登陆框的上边框和左边框的距离（X和Y）
2.鼠标移动的时候，将登陆框的位置设置为可视距离（clientLeft）- 减去上面获取的X或Y = 登陆框的位置
3.鼠标弹起时，移除鼠标移动事件
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.login-header {
width: 100%;
text-align: center;
height: 30px;
font-size: 24px;
line-height: 30px;
}
ul, li, ol, dl, dt, dd, div, p, span, h1, h2, h3, h4, h5, h6, a {
padding: 0px;
margin: 0px;
}
.login {
width: 512px;
position: absolute;
border: #ebebeb solid 1px;
height: 280px;
left: 50%;
right: 50%;
background: #ffffff;
box-shadow: 0px 0px 20px #ddd;
z-index: 9999;
margin-left: -250px;
margin-top: 140px;
/*原来的横轴50%和纵轴50%将登陆框的左上角做到居中了，现在返回长宽的一半*/
display: none;
}
.login-title {
width: 100%;
margin: 10px 0px 0px 0px;
text-align: center;
line-height: 40px;
height: 40px;
font-size: 18px;
position: relative;
cursor: move;
-moz-user-select: none; /*火狐*/
-webkit-user-select: none; /*webkit浏览器*/
-ms-user-select: none; /*IE10*/
-khtml-user-select: none; /*早期浏览器*/
user-select: none;
}
.login-input-content {
margin-top: 20px;
}
.login-button {
width: 50%;
margin: 30px auto 0px auto;
line-height: 40px;
font-size: 14px;
border: #ebebeb 1px solid;
text-align: center;
}
.login-bg {
width: 100%;
height: 100%;
position: fixed;
top: 0px;
left: 0px;
background: #000000;
filter: alpha(opacity=30);
-moz-opacity: 0.3;
-khtml-opacity: 0.3;
opacity: 0.3;
display: none;
}
a {
text-decoration: none;
color: #000000;
}
.login-button a {
display: block;
}
.login-input input.list-input {
float: left;
line-height: 35px;
height: 35px;
width: 350px;
border: #ebebeb 1px solid;
text-indent: 5px;
}
.login-input {
overflow: hidden;
margin: 0px 0px 20px 0px;
}
.login-input label {
float: left;
width: 90px;
padding-right: 10px;
text-align: right;
line-height: 35px;
height: 35px;
font-size: 14px;
}
.login-title span {
position: absolute;
font-size: 12px;
right: -20px;
top: -30px;
background: #ffffff;
border: #ebebeb solid 1px;
width: 40px;
height: 40px;
border-radius: 20px;
}
</style>
</head>
<body>
<div class="login-header"><a id="link" href="javascript:void(0);">点我不花一分钱</a></div>
<div id="login" class="login">
<div id="title" class="login-title">登录会员
<span><a id="closeBtn" href="javascript:void(0);" class="close-login">关闭</a></span></div>
<div class="login-input-content">
<div class="login-input">
<label>老哥名字：</label>
<input type="text" placeholder="请输入用户名" name="info[username]" id="username" class="list-input">
</div>
<div class="login-input">
<label>暗语是啥：</label>
<input type="password" placeholder="请输入登录密码" name="info[password]" id="password" class="list-input">
</div>
</div>
<div id="loginBtn" class="login-button"><a href="javascript:void(0);" id="login-button-submit">登录会员</a></div>
</div><!--登录框-->
<div id="bg" class="login-bg"></div><!--遮挡层-->
<script type="text/javascript">
function myGet(id){
return document.getElementById(id);
}
myGet("link").onclick = function(e){
myGet("login").style.display = "block";
myGet("bg").style.display = "block";
}
myGet("closeBtn").onclick = function(){
myGet("login").style.display = "none";
myGet("bg").style.display = "none";
}
myGet("title").onmousedown = function(e){
var spaceX = e.clientX - myGet("login").offsetLeft;
var spaceY = e.clientY - myGet("login").offsetTop;
// 获取鼠标点击时，点击的位置距离登陆框左边框和上边框的距离
document.onmousemove = function(e){
var x = e.clientX - spaceX + 250;
var y = e.clientY - spaceY - 140;
// 现在登陆框的左边框和上边框的位置，应该为鼠标的可视化的位置
// 减去距离登陆框左边和上边的距离，同时因为登陆框需要居中显示，设置margin值，需要抵消掉
myGet("login").style.left = x + "px";
myGet("login").style.top = y + "px";
}
}
myGet("title").onmouseup = function(){
document.onmousemove = null;
// 鼠标弹起时，清除鼠标移动事件，不再改变位置
}
</script>
</body>
</html>效果：