A large F/number (F/#) cooled infrared optical system based on warm diaphragm was designed.The influences of thermal radiation of the warm diaphragm itself,the external environment stray radiation and the narcissus reflection on the system performance were eliminated by optimizing the design parameters of the warm diaphragm.The system achieved the design requirements of high thermal imaging performance,long focal length and small volume.Performance tests and imaging results show that:the reflective warm diaphragm will not bring significant change to the performance of the system through reasonable optimization design,which verifies the correctness and feasibility of the design method.