<!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 name="viewport" content="initial-scale=1, width=device-width, maximum-scale=1, user-scalable=no">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
</head>
<body>
<style>
#canvas{display:block;margin:100px auto; background:#fff;}
</style>
<canvas id="canvas" width="300" height="300"></canvas>
<script type="text/javascript">
window.onload = function(){
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var W = canvas.width;
var H = canvas.height;
var deg=0,new_deg=0,dif=0;
var loop,re_loop;
var text,text_w;
function init(){
ctx.clearRect(0,0,W,H);
ctx.beginPath();
ctx.strokeStyle="#ebebeb";
ctx.lineWidth=5;
ctx.arc(W/2,H/2,70,0,Math.PI*2,false);
ctx.stroke();
var r = deg*Math.PI/180;
ctx.beginPath();
ctx.strokeStyle = "#f31437";
ctx.lineWidth=5;
ctx.arc(W/2,H/2,70,0-90*Math.PI/180,r-90*Math.PI/180,false);
ctx.stroke();
ctx.fillStyle="#f00";
ctx.font="50px abc";
text = Math.floor(deg/360*100)+"%";
text_w = ctx.measureText(text).width;
ctx.fillText(text,W/2 - text_w/2,H/2+15);
}
function draw(){
new_deg = 250;//设置角度
loop = setInterval(to,1000/dif);
}
function to(){
if(deg == new_deg){
clearInterval(loop);
}
if(deg<new_deg){
deg++;
}else{
deg--;
}
init();
}
draw();
re_loop = setInterval(draw,2000);
}
</script>
</body>
</html>
原文：http://www.cnblogs.com/light-j/p/4626240.html