In order to investigate the influence of temperature on the radius of curvature and the RMS value of the large aperture SiC reflective mirror, a finite element model was established for a 2 m-aperture SiC primary mirror of a space telescope, and the effects of uniform temperature field, axial temperature gradient and radial temperature gradient on the radius of curvature and RMS of the mirror were analyzed. The accuracies of the simulation method and results were verified by experiment theoretical arithmetic. The results show that the influence of temperature gradient on the radius of curvature and the RMS value of the mirror is much greater than the effect of uniform temperature. The change of curvature radius is most sensitive to the axial temperature gradient, and the RMS value of the surface figure is most sensitive to the radial temperature gradient. The change in radius of curvature caused by the 1 ℃ axial temperature gradient is 48 times greater than the change in radius of curvature caused by the same uniform temperature rise. The RMS of surface figure caused by the ±1 ℃ radial temperature gradient can be 202 times larger than that caused by the same uniform temperature rise. The influence of axial temperature gradient and radial temperature gradient on the thermal stability tolerance must be considered in determining the thermal control index of the mirror.