const img = this.canvas.createImage();
img.src = 'https://gw.alicdn.com/tfs/TB1rnPH3ND1gK0jSZFsXXbldVXa-72-72.png';
img.onload = () => {
  this.ctx.drawImage(img, 0, 0, 72, 72); 
  // 改成 this.ctx.drawImage(img, 0, 0, 72, 36) 才是正确的
};