Surface shape measure of large aperture mirror components existing problems of high cost and strict environmental requirements. In this paper, based on the sub aperture stitching detection technology of S-H wavefront sensor, the stitching algorithm is improved, and the mixed stitching algorithm is proposed, which can effectively reduce the error caused by the sub-aperture stitching. With auto-collimation wave-front detection system and high precision two-dimensional scanning system, the reflective surface shape of large aperture reflector is detected by surface stitching. The experimental results show that the measurement error of the device is approximately 0.072 λ, which meets the requirements of the process of the large aperture mirror assembly from the high power solid laser.