参考网上的一些资料后，总结出来  

<!DOCTYPE html>
<html>

<head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <title>图片下载</title>
</head>

<body>
  <h1>图片下载</h1>
  <script>

      function downloadImage(){

          var CAPTURE_SCREEN_FILE_NAME = 'download.jpeg';//文件名

          if(typeof window.navigator.msSaveBlob !== 'undefined') {
              window.navigator.msSaveBlob( document.getElementById("kvm").msToBlob() /* Canvas to Blob */,
                      /* Save file name */ CAPTURE_SCREEN_FILE_NAME );
              return;
          }

          // Works as fall back mechanism, in case of browsers other than IE.
          var link = document.createElement('a');
          // We wan't to save the canvas to JPEG format. So giving parameter value for toDataURL() method, as "image/jpeg".
          // If not mentioned, canvas will be exported as PNG format.
          var canvas=document.getElementById('kvm');
          var ctx=canvas.getContext('2d');
          var img=new Image();
          img.src='image/2.jpg';//图片url,可以是本地也可是网络或后端url
          img.onload=function () {
              ctx.drawImage(img,0,0);
          }
          link.href  = document.getElementById("kvm").toDataURL("image/jpg");
          link.download = CAPTURE_SCREEN_FILE_NAME; // Save file name
          document.body.appendChild(link); // For Firefox browser
          link.click(); // Start Download
          link.remove(); // For Firefox browser
      }
  </script>

  <canvas id="kvm"></canvas>
  <!--<img id="kvm"  src="image/2.jpg" />-->
  <input type="button" onclick="downloadImage()" value="下载" />



</body>

</html>