A diameter of 1.2 m space mirror and its supporting structure were designed. Firstly,Materials of the mirror and the support were reasonably selected. Taking account of the factor of technology,mechanical property,thermal property and lightweight,RB-SiC was chosen as the material of mirror and a new design with back semi-closed sector reinforced ribs lightweight structure was proposed,weight of mirror is 87.6 kg and lightweight ratio reaches to 79%. Secondly,mirror with the back three-point support scheme was determined,through the rational design of flexible support structure parameters to ensure high stiffness and thermal dimensional stability. Finally,through using finite element analysis,the mirror subassembly was analyzed in detail. Analysis results indicate that the surface figure accuracy RMS can reach 7.8 nm and 9.4 nm under gravity and 4℃ uniform temperature rise respectively,and the first order natural frequency is 153 Hz,which can meet the design requirements.