我用canvas载入图片：

wx.createSelectorQuery().select('#myCanvas').fields({ node: true, size: true }).exec((res)=>{
      const canvas = res[0].node
      that.canvas = canvas;
      that.context = canvas.getContext('2d')
      canvas.width = res[0].width * app.globalData.pixelRatio
      canvas.height = res[0].height * app.globalData.pixelRatio
      const img = canvas.createImage();
      img.src = that.data.url;
      img.onload = () => {
        //计算图片和canvas的宽高比，对图片进行保持比例缩放。图片可正常按比例载入
        that.context.drawImage(img, 0, 0, that.data.scaledWidth/**计算后的图片宽**/, that.data.scaledHeight/**计算后的图片高**/);
      }
}
confirmTap: function () {
    var that = this;
    wx.showLoading({
      title: '正在保存图片',
    })
    setTimeout(() => {
        wx.canvasToTempFilePath({
          x: 0,
          y: 0,
          width: that.data.scaledWidth,
          height: that.data.scaledHeight,
          destWidth: that.data.scaledWidth * app.globalData.pixelRatio,    /**app.globalData.pixelRatio为像素比**/
          destHeight: that.data.scaledHeight * app.globalData.pixelRatio,
          // width: 10,
          // height: 10,
          // destHeight: 300,
          // destWidth: 300,
          fileType: 'jpg',
          canvas: this.canvas,
          success: function (res) {
            console.log(res);
            var tempFilePath = res.tempFilePath;
            console.log(tempFilePath);
            wx.saveImageToPhotosAlbum({
              filePath: res.tempFilePath,
              success:function(res){
                wx.hideLoading()
              }
            })
          }
        })
      }, 1500);
}

但实际保存的图片是整个canvas的宽高，但被缩放成that.data.scaledWidth * app.globalData.pixelRatio和that.data.scaledHeight * app.globalData.pixelRatio

加载时的图片

保存的图片