In order to explore the feasibility of large ultra-lightweight deployable optical systems, a 1.25m SiC segmented mirror is investigated. According to analysis and comparison, the mirror's material, ultra-lightweight structure pattern and support location are determined respectively. By FEM, an ultra-lightweight structure with areal density of 40kg/m2 is gotten. The results show that the self-weight deformation is 4.8nm RMS/22.6nm PV under supports, and the ultra-lightweight mirror has the enough strength to bear the stress at launch. The study may provide a technical scheme to develop the large ultra-lightweight deployable optical system.