As an important part of the optical system, the accuracy of the plane mirror is an influence factor to system imaging. Subaperture stitching testing is a usual way to test plane mirror in large aperture, while the stitching algorithm is the key in the stitching technology. The plane sub-aperture stitching algorithm was studied in the paper and a reasonable stitching algorithms and mathematical models was established based on maximum likelihood estimation and orthogonalization Zernike polynomial fitting. Stitching to plane mirror in large aperture can be accomplished with the above stitching model. Stitching program was also written and stitching testing was carried on with a Φ100 mm interferometer on a Φ120 mm plane mirror. Comparing the stitching result with the full aperture testing result, it shows that the stitching map is in consistent with the full aperture testing map. The difference of RMS between them is 0.002λ, verifying the reliability and accuracy of the algorithm.