In order to solve the problem of large optical convex spherical mirror testing, a stitching model was established, which was based on global optimization algorithm and least-square method. The full aperture map can be achieved with this relative algorithm. At the same time, based on our model, aφ120 mm convex spherical mirror was tested. In the testing, five subapertures were tested. It can be seen that the stitching map is smooth and continuous, with no stitching mark. To evaluate the stitching accuracy, a subaperture called self-calibration subaperture was tested. Comparing the stitching map with the self-calibration map point by point, the residual map can be got. It can be seen from the residual map that the PV value is 0.014λ and the RMS value is 0.003λ, proving the reliability and accuracy of the stitching algorithm.