A non-contact measuring method for testing the flatness of a mirror or mirror-like surface by reflection of a laser beam is mentioned. A laser beam breaks into several beams arranged at constant intervals and in the same direction, and these beams are projected on a mirror surface to be measured. The reflecting beams thus obtained are intercepted by a screen separately provided, and by measuring the coordinates of the spots of light thus obtained, the deviation compared with the ideal geometrical plane at each reflecting point is calculated. In this paper, the authors propose the measuring principle, measuring errors and experimental results of the above method, and make it clear that the flatness of the mirror can be obtained mechanically by this method.