Abstract: Stray light suppression in a catadioptric and re-imaging LWIR system was discussed. The optical system which comprised a classical R-C reflective optics and a re-imaging refractive optics was designed for a compact and low-cost ⅡR seeker. The source and path of stray light in this system were investigated by simulating the optic-mechanical model with LightTools. According to the simulation, the mechanical light stops were built, and the performance of stray light suppression was analyzed. After assembled, the optical system and its anti-stray light effect were evaluated by testing the angle limit away from the sun. The results of simulation and test indicate that there is no influence of stray light on the LWIR system outside 7 from optical axis.