Abstract: To the testing of large-diame ter optical mirror, with the increase in the size of the reference mirror, it can't be treated as an ideal plane mirror because of the constraints of the machining accuracy, gravity deformation, temperature, environment impact. Based on the maximum likelihood estimation, Zernike polynomials fitting, a reasonable stitching algorithms and mathematical models was established for non-ideal standard lens sub-aperture stitching. Stitching experiment was accomplished on the computer with a 2.5m3.5m oval plane mirror as an engineering example. The full-aperture surface error distribution after stitching was consistent with the original. PV and RMS deviation were 0.022 and 0.001 3 . The full aperture phase distribution of the PV value and the RMS value of relative error was 2.81% and 0.81% respectively. The experimental results show that: large-diameter plane mirror can be tested with non-ideal reference mirror in high precision using the ML algorithm.