Abstract: In Ritchey-Common test，the intrinsic astigmatism of the mirror plane under test and the astigmatism of the large curvature can not be distinguished in the measured system wave-front aberration. The flat is set in a divergent beam, which makes the relation between figure errors of flat and wavefront aberrations (influence function) complicate and difficult to derive. The procedure to simulate the Ritchey-Common test to derive the relationships between the figure errors of an optical flat and the wavefront aberrations, as well as the procedure to construct two set of influence functions by ray-tracing program are presented. Figure errors of the flat can be extracted from a set of over defined linear equations with the aid of least squares, using the influence functions and the measured wavefront aberrations for two different Ritchey angles. Both Ritchey-Common test and direct measurement results are presented. The comparison result proved the feasibility and reliability of this method．