For the optical system using refrigeration mid-wave infrared detectora multi-spectral mid-wave infrared convergence system is designed.To solve the problem of external stray radiation introduced by color separation filtera thermal radiation suppression stop is designedand its shape and position are optimized through iterationso as to suppress the external stray radiation of the system.The TracePro simulation shows that this thermal radiation suppression stop can reduce the total stray radiation to 8.8%.It has obvious effects of improving the imaging quality.