downloadImg() {
            let down = plus.downloader.createDownload('图片的URL', {}, function (e, a) {
                plus.gallery.save(
                    e.filename,
                    function (e) {
                        uni.showToast({
                            title: '保存成功',
                            mask: true
                        });
                        console.log('下载成功');
                    },
                    function (e) {
                        console.log('下载失败,请重试');
                    }
                );
            });
            down.start();
  }

搞定！