Stray light control is an important technical indicator of an optical system’s performance. The stray light in infrared optical systems includes not only external stray light, but also the internal radiation stray light. The conventional initial system design method limited to an “object-image" conjugate relationship cannot properly take account of stray light. This paper presents a triple conjugate optical system design method for “object-image”, “object-intermediate real image” and “entrance pupil-exit pupil” triple conjugate relationships that minimizes the influences of internal and external stray light to provide good stray light control performance. With this method, an off-axis three-mirror anastigmatic infrared optical system with F/# = 4, linear field of view = 7°, point source transmission less than 5×10-4, and cold iris efficiency 96% was designed and imaged good pictures in orbit.