Abstract: TMT (Thirty Meter Telescope) is a thirty meter Ritchey-Chr佴tien optical-infrared telescope. The Third Mirror (M3) is an elliptical flat mirror, whose aperture is 3.594 m2.568 m, and the weight is 1.8 ton. The M3 system reflects the light coming from the Secondary Mirror System to the science instruments located on the Nasmyth platforms, so the M3 system rotates and tilts as the telescope rotates about the elevation axis to track astronomical objects across the sky. Due to the M3 support system the mirror figure maintains a high precision. The specification requires that the RMS of the mirror surface figure will be less than /5(=632.5 nm) or the SlopeRMS will be less than 1rad. This paper took one of the conceptual designs as an example to illustrate the method of evaluating the SlopeRMS. The axial support system in this plan used an 18 points whiffletree structure while the lateral support system used an 8 points A-frame structure. The FE model was constructed in ANSYS. Then MATLAB was employed to carry out the data processing. In the data processing program, the rigid movement was removed at first. Then the elliptical mirror was normalized into a circle mirror and the defocus and the astigmatism were eliminated. In the end, the SlopeRMS of nine sub -apertures was calculated. This method for evaluating the mirror surface figure is different from the classic one for the primary mirror, but it is effective to evaluate the elliptical mirror surface figure indeed. This method is meaningful in mirror surface figure evaluation.