1.onmousemove事件触发时不断更新鼠标的pageXY改变位置，
登陆框的偏移量=鼠标当前位置-鼠标到登录框边框的距离
2.onmousedown鼠标摁下时触发事件获取鼠标到登陆框的距离，再设置true允许拖拽
3.onmouseup 鼠标弹起设置false停止拖拽
4.登录框居中显示公式：(可视区域宽高-登录框宽高)/2
5.当浏览器窗口大小变化时触发事件window.onresize 再更新登陆框居中显示
代码中有详细的注释
完整代码
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>demo</title> <style> body,h1,h2,h3,h4,h5,h6,hr,p,blockquote,dl,dt,dd,ul,ol,li,pre,form,fieldset,legend,button,input,textarea,th,td{margin:0;padding:0;} body,button,input,select,textarea{font:12px/1.5 tahoma,arial,\5b8b\4f53;} h1,h2,h3,h4,h5,h6{font-size:100%;} address,cite,dfn,em,var{font-style:normal;} code,kbd,pre,samp{font-family:courier new,courier,monospace;} small{font-size:12px;} ul,ol{list-style:none;} a{text-decoration:none;} a:hover{text-decoration:underline;} sup{vertical-align:text-top;} sub{vertical-align:text-bottom;} legend{color:#000;} fieldset,img{border:0;} button,input,select,textarea{font-size:100%;} table{border-collapse:collapse;border-spacing:0;} .clear{clear: both;float: none;height: 0;overflow: hidden;} /*p{font-size: 100px;}*/ #btn{width: 80px; height: 40px; background: #3b7ae3; margin:0 auto; display: block; cursor: pointer; border-style: none; color: #fff; font-size: 16px;} #mask{ background: #000; opacity: 0.75; filter: alpha(opacity=75); height: 1000px; width: 100%; position: absolute; left: 0; top: 0; z-index: 1000; } #login{position: absolute; top: 100px; left: 100px; width: 400px; height: auto; border:1px solid #d5d5d5; z-index: 1001; } .title{position: relative;background-color: #f7f7f7; cursor: move; height: 50px; line-height: 50px; font-size: 16px; color: #333; padding-left:30px;} .close{position: absolute; top:0; right: 10px; color: #ccc;} .content{background: #fff; padding: 15px 20px;} .user{margin-bottom: 15px;} .password{margin-bottom: 15px;} .pt{display: block; height: 38px; padding-left: 15px; border: 1px solid #ddd; transition: .3s; font-size: 14px; color: #666; width: 343px; } .sm{display: block; height: 48px; border: 1px solid #ddd; transition: .3s; font-size: 16px; color: #666; width: 360px; background: #3b7ae3; color: #fff;} </style> </head> <body> <!-- <div></div> --> <button href="">登录</button> <!-- <div> <div>登录百度账号<a href="#">x</a></div> <div> <div><input type="input" value="手机/邮箱/用户名"></div> <div><input type="input" value="密码"></div> <div><input type="submit" value="登录"></div> </div> </div> --> <script type="text/javascript"> function b(){ //创建遮罩层div并插入body var mask=document.createElement("div"); mask.id="mask"; mask.style.height=cheight+"px"; //宽度直接用100%在样式里 document.body.appendChild(mask); //创建登录层div并插入body var login=document.createElement("div"); login.id="login"; login.innerHTML='<div>登录百度账号'+'<a href="#">x</a>'+'</div>'+ '<div>'+'<div>'+'<input type="input" value="手机/邮箱/用户名">'+'</div>'+'<div>'+'<input type="input" value="密码">'+'</div>'+'<div>'+'<input type="submit" value="登录">'+'</div>'+'</div>'; document.body.appendChild(login); //窗口可视区域宽度 var cwidth= document.documentElement.clientWidth || document.body.clientWidth; //窗口可视区域高度 var cheight= document.documentElement.clientHeight || document.body.clientHeight; //登录框宽度 var lwidth=login.offsetWidth; //登录框高度 var lheight=login.offsetHeight; //设置登录框的居中显示 login.style.left=(cwidth-lwidth)/2+"px"; login.style.top=(cheight-lheight)/2+"px"; //设置遮罩层的高度 mask.style.height=cheight+"px"; //改变窗口大小后依然居中显示 window.onresize=function(){ if(document.compatMode=="CSS1Compat"){ cwidth=document.documentElement.clientWidth; cheight=document.documentElement.clientHeight; }else{ cwidth=document.body.clientWidth; cheight=document.body.clientHeight; } login.style.left=(cwidth-lwidth)/2+"px"; login.style.top=(cheight-lheight)/2+"px"; mask.style.height=cheight+"px"; } //获取拖拽容器 var title=document.getElementById("title"); var isDraging=false; var mouseOffsetX; var mouseOffsetY; //鼠标按下事件 title.onmousedown=function(e){ var e=e||window.event; /*var el=e.srcElement; if(!el){ el=e.target;//兼容火狐 }*/ //鼠标相对于登录框的位置 mouseOffsetX=e.pageX-login.offsetLeft; mouseOffsetY=e.pageY-login.offsetTop; //鼠标摁下时为true isDraging=true; /*console.log(mouseOffsetY, mouseOffsetX)*/ } //鼠标移动事件 document.onmousemove=function(e){ var e=e||window.event; //鼠标移动时的坐标 var newMX=e.pageX; var newMY=e.pageY; //判断为true时可以拖拽 if(isDraging===true){ //登录框的偏移值=当前位置-鼠标到登录框的距离 var loginL=newMX-mouseOffsetX; var loginT=newMY-mouseOffsetY; //如果left top值超过边缘时就让他等于边缘 if(loginL<0){ loginL=0; }else if(loginL>(cwidth-lwidth)){ loginL=cwidth-lwidth; } if(loginT<0){ loginT=0; }else if(loginT>(cheight-lheight)){ loginT=cheight-lheight; } login.style.left=loginL+"px"; login.style.top=loginT+"px"; } } //鼠标弹起时设置为不可拖拽 document.onmouseup=function(){ isDraging=false; } //点击X关闭登录框和弹出层 var close=login.getElementsByClassName("close")[0]; close.onclick=function(){ document.body.removeChild(mask); document.body.removeChild(login); } } //点击登录弹出登录框和弹出层 window.onload=function(){ var btn=document.getElementById("btn"); btn.onclick=function(){ b(); } } </script> </body> </html>