During the testing of aspherical mirrors through the null compensation method, structural parameters observe changes which affect the test results of interference pattern. Several errors are induced due to the maladjustment among compensator, interferometer and the mirror under test. It is important for optical manufacturing, testing and the actual alignment process to distinguish between the aberrations arising from both surface errors and maladjustment of the null compensation testing system. The purpose of this paper is to obtain the real aspheric surface errors during the optical polishing process. In this work, we have established an error separation model to the least square method to separate the errors caused by the maladjustment of the testing system from the test results. Finally, the analysis and simulation results show that high precision figure errors can be obtained by separating the maladjustment errors.