Abstract: Improved hydraulic whiffletree passive support system was designed for 2 m SiC lightweight primary mirror. The lever counterweight mechanism connected to hydraulic whiffletree mechanism was used for axial support system. The axial support force optimization distribution in different support points was accomplished by the axial support system, and the root-mean-square (RMS) of the mirror distortion resulted by axial support system was improved from 7.1 nm to 4.8 nm, which was optimized by finite element analysis (FEA) method. Considering the large thermal expansion coefficient of SiC, tangent link mechanism which owned adaptive bucking thermal stress attribute combined with hydraulic whiffletree mechanism was applied to the lateral support system for SiC primary mirror. When telescope was printed to horizon, the mirror distortion caused by the lateral support system was analyzed, and 39.7 nm RMS of the primary mirror deformation was obtained by FEA. As the ambient temperature raised from 20 ℃ to 40 ℃, the mirror distortion resulted by the axial support and lateral support system was still kept on the level of 4.8 nm RMS. Analysis result indicates well thermal adaptability of the improved lateral support system.