As infrared radiation is weakened by infrared stealthy technology，the infrared radiation characteristics of aircraft affected by infrared stealthy coating is analyzed based on conversation of energy. The atmospheric transmission with different paths is calculated by Modtran software，and the transmission rule which is propitious to infrared detection is discussed. Two factors including Signal-to-Noise and Aperture of optical system affected on operating range are also analyzed by using the generalized range equation of IR system. The calculations indicate that with low Signal-to-Noise and large aperture of optical system，the operating range for stealth aircraft detected by IR system can be more than 250 km.