Abstract: Stray radiation refers to the unwanted radiation reached on the detector surface in infrared detection system. Stray radiation will add the system's noise after photoelectric conversion, decrease the system's output signal-noise-ratio, and reduce the system's detection ability for space target. In order to improve the detection ability of infrared system for space dim and small target, the sources of stray radiation in infrared detection system were analyzed. The formula of the sun's incident angle on infrared detection system at certain geographic latitude and time was deduced. The equivalent number of noise electrons from the sun, sky and the infrared detection system's thermal radiation was calculated based on a typical system parameter. The noises arisen from stray radiation and detector's inherent noise were compared, the noises limit of infrared detection system was analyzed. Results prove that noise limit of infrared detection system is background noise after analysis of the system designed.