In order to realize the lightweight of the (165mm×96mm) rectangular scanning mirror assembly and ensure the accuracy of the mirror shape and the support stiffness of the assembly, a back support method with the integration of cone-sleeve and flexible joints was proposed, and the weight of the mirror assembly designed with this method is less than 0.5kg. The mirror body was made of silicon carbide and the supporting structure was made of indium steel. The support stiffness and surface accuracy of the scanning mirror assembly are simulated and analyzed by finite element analysis and verified by actual testing. The results show that the maximum root mean square (RMS) value of the scanning mirror shape error is 9.705nm, the actual test result 10.125nm, the error is 4% and meets the requirement that the RMS value should be less than 12.6nm in the case of gravity in all directions and the torque of the shaft-driven. The first-order natural frequency of the component was 302.25Hz, which met the stiffness requirements. The results show that the proposed method is reasonable and effective, which solves the problem that the ultra-lightweight structure and the structural stiffness and optical surface accuracy are difficult to guarantee at one time.