To solve the problem of high-precision testing of large-diameter plane mirrors, a mathematical model of subaperture stitching testing based on global optimization was established, and a stitching factor was proposed for overlapping area values. Based on the above method, combined with engineering examples, the stitching testing of a plane mirror was completed with a diameter of 120 mm, and four subapertures to be tested were planned. In order to compare the stitching performance of the algorithm described in this paper with the traditional least-squared fitting stitching algorithm, two algorithms were used to complete the surface reconstruction of the plane mirror to be measured. The experimental results show that the stitching results obtained by the two algorithms are smooth, continuous, no "stitch marks". At the same time, the results of the two algorithms are also compared with the full-aperture testing results. In this paper, obvious "stitch marks" can be seen in the residual map of the traditional splicing algorithm, and the stitching results obtained by the algorithm method in this paper are smooth and continuous, while the PV and RMS values of the residual graph are 0.012λ and 0.002λ, respectively, which are less than the PV and RMS values of the traditional algorithm residuals chart, which verifies the reliability and accuracy of the algorithm.