Abstract: Stray light analysis in optical system is a key technology to ensure the system's quality. According to the definition of stray light, the sources of stray light in infrared optical system and the effect of stray light were pointed out in this paper. In view of stray light source, a structure of suppressing stray light was established. In order to reduce the internal radiation, reflective vanes were used. TracePro software was also used to build the system model and simulate analysis of the system. The results indicate that stray light is well suppressed, and the system's PST can achieve a level of 10-5-10-8. At the same time, the internal radiation energy that gets to imager can achieve a level of 10-10 W. So the system can achieve a clear image.