diff --git a/11_day/sheet.c b/11_day/sheet.c
index e5bd624..2df8d99 100644
--- a/11_day/sheet.c
+++ b/11_day/sheet.c
@@ -113,6 +113,13 @@ void sheet_refreshsub(struct SHTCTL *ctl, int vx0, int vy0, int vx1, int vy1)
 	int h, bx, by, vx, vy, bx0, by0, bx1, by1;
 	unsigned char *buf, c, *vram = ctl->vram;
 	struct SHEET *sht;
+
+	/* 如果refresh的范围超出了画面则修正 */
+	if (vx0 < 0) { vx0 = 0; }
+	if (vy0 < 0) { vy0 = 0; }
+	if (vx1 > ctl->xsize) { vx1 = ctl->xsize; }
+	if (vy1 > ctl->ysize) { vy1 = ctl->ysize; }
+
 	for (h = 0; h <= ctl->top; h++) {
 		sht = ctl->sheets[h];
 		buf = sht->buf;