In order to meet the requirements of ultra-lightweight high precision space optical equipment, an ultra-lightweight structure design method of Φ600mm mirror assembly combined with multi-objective integrated optimization and secondary sintering process was proposed by using parameter optimization technology, finite element analysis and advanced SiC manufacturing technology. Firstly, the root mean square value (RMS) and weight of the sic mirror shape in the X and Z directions were taken as the optimization objective, and the axial thickness of the mirror and the thickness of the mirror surface were taken as the design variables, the ultra-lightweight mirror structure with a lightweight rate of 90.55% was obtained. Secondly, the mirror supporting structure is also made of SiC material, the mirror and support are directly sintered by using the secondary sintering process method to reduce the bonding assembly link, and the mirror assembly with a lightweight rate of 92% is obtained. Finally, using the finite element analysis and test of ultra-low weight the correctness and rationality of the structure design method is verified. The results show that under the comprehensive influence of gravity load, temperature load and mirror machining residual, the RMS value of mirror assembly surface error is 10.034nm, which is better than the design requirements of 12.6nm, and the dynamic stiffness is good, which meets the requirements of use.