With the development of advanced optical system design and manufacturing, large aperture optical system has been widely used. However, the lack of high precision surface shape detection means limits the manufacture and application of large aperture plane mirrors. In order to detect the surface shape of large aperture planar mirror with high precision, a Shack-Hartmann scanning and stiching detection method was proposed. The scanning and stiching principle and wavefront reconstruction algorithm were studied, and the mathematical model of microlens array imaging was established to verify the feasibility of Shack-Hartmann scanning and stiching detection principle. A scanning and stiching test experiment was carried out for a 150 mm aperture plane mirror, the full aperture surface shape 0.019λRMS(λ=635 nm) was obtained. Compared with the results of interference detection, the detection accuracy was 0.008λRMS. The results show that the method can realize the high precision detection of large aperture planar mirror.