<a href="图片src" download="下载海报">
下载海报
</a>

img.crossOrigin = 'anonymous';

canvas.toDataURL("image/png")

 var save_link = document.createElement('a');
     save_link.href = image;
     save_link.download ='测试.png';
                           
 var clickevent = document.createEvent('MouseEvents');
     clickevent.initEvent('click', true, false);
     save_link.dispatchEvent(clickevent);

var canvas = $('.canvas');
var cxt = canvas[0].getContext("2d");
function save(){
    var img = new Image();
    img.crossOrigin = 'anonymous';
    img.onload = function(){
        var _w = img.naturalWidth;
        var _h = img.naturalHeight;
        canvas.attr({width:_w,height:_h});
        cxt.drawImage(img,0,0);
        var image = canvas[0].toDataURL("image/png")
        var save_link = document.createElement('a');
        save_link.href = image;
        save_link.download ='测试.png';
                           
       var clickevent = document.createEvent('MouseEvents');
           clickevent.initEvent('click', true, false);
           save_link.dispatchEvent(clickevent);
    };
    img.src = 'http://n.sinaimg.cn/sports/transform/20170825/NZI3-    fykiufe6650492.jpg';
}