Due to the limited load of aircraft, weight was always the key index in the structural design of airborne imaging system. As the main load bearing structure of the optical system in the airborne imaging system, the main support structure must be lightweight. However, the previous lightweight design methods for the main support structure of airborne imaging system mainly included specific measures such as selecting metal materials with high specific stiffness, optimizing the layout of the frame structure, adjusting the wall thickness, and adding weight loss trough. Due to the high density and linear expansion coefficient of metal materials, the lightness extent of this lightweight design method was not high, and sometimes couldn’t meet the requirements of athermalization design of high-precision optical systems. Therefore, a new lightweight design method combining composite materials and metal materials was proposed. Carbon fiber composite materials with lower density and lower linear expansion coefficient was used as the main support molding material, and titanium alloy was used as the external interface material. The parameter optimization design was carried out with the lightest target as the goal and the fundamental frequency as the constraint. Finally, the main support structure with higher lightweight and better dimensional stability was obtained by using the prepreg manufacturing and laying method. The effectiveness of the new method was verified by numerical calculation, simulation analysis and vibration test. The results showed that the fundamental frequency of the new lightweight main support system was 425 Hz. The weight was 10.5 kg, which was reduced by 33.5%. The variation of axial optical spacing was 0.021 mm at 60 ℃ uniform temperature rise, which was reduced by 84.9%. The research results showed that the new lightweight design method was reasonable and effective, which solved the problem of structural lightweight and optical athermalized design. It was applied to the main support structure of the airborne infrared imaging system.