A method has been put forward for design small-angle scanning mirrors to increase the scanning frequency of broad beam scanner, and experiment with this designed scanning mirror has been carried out. The size of the elliptical spot irradiated by the scanning beam is worked out according to the size of the beam and the scanning angle. Based on the ellipse inscribed, a suitable octagonal mirror is produced as the scanning mirror. We experimented with the reflection mirror designed ourselves loaded on M-6870 scanner produced by the Cambridge Technology Inc. The result indicates a small-angle mirror could increase the scanning frequency to a great degree.