diff --git a/lib/pen.js b/lib/pen.js
index 4a8c5fa..c4e803b 100644
--- a/lib/pen.js
+++ b/lib/pen.js
@@ -34,7 +34,7 @@ export default class Painter {
       // 如果未设置背景，则默认使用白色
       this.ctx.setFillStyle('#fff');
       this.ctx.fillRect(-(width / 2), -(height / 2), width, height);
-    } else if (bg.startsWith('#') || bg.startsWith('rgba')) {
+    } else if (bg.startsWith('#') || bg.startsWith('rgba') || bg.toLowerCase() === 'transparent') {
       // 背景填充颜色
       this.ctx.setFillStyle(bg);
       this.ctx.fillRect(-(width / 2), -(height / 2), width, height);