A typical infrared detection system [Ritchey-Chretien (R-C) system] and its baffles are designed.Its structure model is established with the TracePro software.Stray lights caused by thermal radiation of critical inner surface are analyzed by a ray tracing program.The relation of photons of stray lights and temperature which varies from 250 K to 320 K is given.The stray light caused by sky background is calculated.The curve of increasing rate of photon number of stray lights received by single pixel versus temperature is given.The results show that an open telescope without main tube cannot do well obviously in reducing stray lights.The temperature of the inner baffle of main mirror should be less than 230 K in order to detect the background-limited target.