Rational lightweight design is the practical and effective method which can reduce the weight of large aperture mirror and decrease the influence of deadweight and other deformation on the surface figure. The 1 200 mm aperture primary mirror is taken as the research object. According to the principle of equal rigidity and considering current process technology, two lightweight design schemes are worked out. Based on the results of finite element analysis, the structure with RB-SiC technology, semi-closed back, partial cone rear surface and sector hole are judged to be the preferred scheme at last, and the primary mirror is manufactured successfully.