Abstract: In order to evaluate the plane mirror surface shape error, the surface shape detection principle based on the fringe reflection method was adopted, and the method for realizing the plane shape error evaluation by the plane mirror normal offset was used. The sine fringe was projected on the light screen, reflected by the plane mirror to the camera, and the normal of a point on the plane mirror was obtained through the imaging principle of the small hole and the four-step phase shift method. And this point was reconstructed by the least square algorithm surface shape. Theoretical normal comparison was used to obtain the normal offset of the point, and the surface shape evaluation of the plane mirror was achieved by using the normal offset. Computer simulation of the measurement method to prove its correctness; set up an experimental measurement system, and perform measurement experiments on a certain mobile phone screen. The results show that the repeatability accuracy of this measurement method is better than 1mrad, and the measurement repeatability is high. This study provides a reference for the flat surface shape error of glass panels and other products with high reflection characteristics.