According to the design requirement of a certain primary mirror assembly in Cassegrain system,based on the principle that the thermal expansion coefficient of the support structure should match with that of the mirror,three primary mirror assembly schemes are designed and analyzed by finite dement analysis and parameter optimization method. The results show that with the same structural parameters the SiC mirror has the better static and dynamic properties than the fused silica mirror and the K9 mirror. After the lightweight structure optimization,the mass of the SiC mirror with 256 mm aperture is only 1.22 kg, The structure/thermal analysis of the three schemes indicate that the third scheme,where both of the mirror and the support structure in the primary mirror assembly are made by the SiC material,has the better over-all properties than that of the other schemes, Moreover,the thermal expansion mismatch between the materials can be eliminated,and the SiC mirror surface is satisfactory with the design requirements in the third scheme.