To apply SiC mirror in a ground-based telescope, a flexible passive support system was designed on its high thermal expansion coefficient and high rigidity. In this system, a thin and long flexible rod was used in the axial support whiffletree structure and a flexure joint rod wad used in the lateral support structure. By which, the axial support structure and the lateral support structure could support the system respectively. This structure not only allows the primary mirror to be a good position and a shape but also easily eliminate assemble stress and thermal stress. The static analysis, thermal analysis and modal analysis for the mirror were performed under the support system, and then the analytical results were verified by a surface test, mirror inclination test and a displacement test. The test result of the primary mirror shows that the surface error(RMS) is λ/40(the optical axis is vertical) and that (RMS ) is λ/16 (the optical axis is horizontal), the maximum inclination change of the primary mirror at different pitch angles is 8 "and the displacement is 0.070 7 mm.The test results are basically consistent with the analysis results, meeting the design requirements. It shows that this flexible passive support system has good engineering feasibility.