A 1.2 m SiC primary mirror FEA model was built to analyze temperature and support style effects on the large aperture SiC primary mirror, which was applied to the ground based on telescope. With the primary mirror in passive support and free expansion, influences of steady state temperature field, axial direction temperature gradient, radial direction temperature gradient, and internal-external temperature gradient to primary mirror surface figure were analyzed. The result indicates that the influences of the support style are not obvious, whether passive support or free expansion, and the primary mirror surface figure is big with temperature gradient. When achieving heat balance, the influence of the support style is obvious only when the primary mirror free expand, and the temperature influence is small, surface figure RMS<0.02nm/℃. If the primary mirror adopts flexibility or floating support style, the large aperture SiC primary mirror can be applied to the ground based telescope.