js代码
async getImage (url,width, height) {  
    const off = wx.createOffscreenCanvas({type:'2d'})
    const image = off.createImage()   
    await new Promise((resolve, reject)=>{      
      image.onload = resolve  // 绘制图片逻辑
      image.src = url
    })
    return image
 
  },
    aaa(){
      const now = new Date();
      const year = now.getFullYear();
      const month = now.getMonth() + 1;
      const day = now.getDate();
      const date = `${year}年${month}月${day}日`;
      const no = `F${year}182006`;
      const that=this;
      wx.createSelectorQuery().select('#myCanvas').fields({ node: true, size: true }).exec((cres) => {
        //在这里可以下获取到canvas组件了，然后创建成画板画布。
          console.log(cres);
          var rect = cres[0]
          console.log(rect)
          var textCanvas = rect.node  // 重点1
          var ctx = textCanvas.getContext('2d')  // 重点2//创建画布
          that.getImage("../../images/card.jpg",800,1000).then(image=>{
            ctx.drawImage(image, 0, 0,1000,800)
          })
          ctx.font ='28px';
          ctx.fillStyle='#000';
          ctx.fillText(`同志在 ${year} 年 1 月 1 日至 ${month} 月 ${day} 日参加了`, 240, 380);
          ctx.fillText(`编号:${no}`,170 , 640);
          ctx.fillText(`${date}`, 670, 640);
          wx.canvasToTempFilePath({
            canvas:textCanvas,
            success: (res) => {
              console.log('成功生成临时文件路径：', res.tempFilePath);
              wx.downloadFile({
                url: res.tempFilePath,
                success: (downloadRes) => {
                  if (downloadRes.statusCode === 200) {
                    console.log(downloadRes)
                    wx.saveFile({
                      tempFilePath: downloadRes.tempFilePath,
                      success: (saveRes) => {
                        console.log(saveRes)
                        console.log('证书保存路径：', saveRes.savedFilePath);
                        wx.showToast({
                          title: '证书下载成功',
                          icon: 'success',
                        });
                      },
                      fail: (err) => {
                        wx.showToast({
                          title: '证书下载失败'+err,
                          icon: 'none',
                        });
                      },
                    });
                  }
                },
                fail: (err) => {
                  wx.showToast({
                    title: '证书下载失败'+err,
                    icon: 'none',
                  });
                  console.log(err);
                },
              });
            },
            fail: (err) => {
              console.error('生成临时文件路径失败：', err);
              wx.showToast({
                title: '证书生成失败',
                icon: 'none',
              });
            },
          });
        })
    },

wxml:
<canvas id="myCanvas" type="2d" style="width:1000px;height:800px"></canvas>

结果：