A circular primary mirror of an off-axis three mirror anastigmat space camera with an aperture of 610 mm was ultra–lightweight designed to meet the requirement of designing lighter and better satellite cameras. By choosing the back-centre-single point supporting way, using the variable rib thickness and the variable rib height sturctural style and combining with the integrated optimization design method, the mirror was designed with a weight of only 6.23 kg and the surface density about 21.3 kg/m2. The support structure for the mirror was designed and the static and dynamic performances of the mirror subassembly were simulated. The simulation results show that the surface figure error (RMS value) is less than 6nm when the mirror subassembly is under the load condition of gravity in three different directions and is less than 1nm when the temperature variation is within ±4℃. The first-order natural frequency of the mirror subassembly is 112 Hz. The results of frequency response analysis present that the maximum stress in the screw hole of the flexible structure is below the yield strength of its material. All performance indexes of the ultra-lightweight mirror designed in this paper meet the requirements of design and the maximum aperture of the mirror which can be supported by single point in centre is 610 mm. The design proposes a method and reference to ultra-lightweight design of same type mirrors.