Abstract: The irradiance contrast formula of target and background for satellite-borne infrared detector under both ground and atmosphere background was derived. The absolute contrast and the relative contrast at various target altitude and temperature were calculated. The analysis conclusions show that the absolute contrast and the relative contrast increase with the target temperature and altitude for wavelength of larger than 2.5 m. The higher the target altitude is, the less the contrast changes per unit distance. The effect of the sun should be considered when the wavelength is less than 2.5 m. For target at 0 km altitude, 2.9-4.1 m is the optimum detective waveband. As the target altitude increases, the available detective waveband broadens and the peak wavelength moves to short wave direction slightly. When the target altitude is higher than 10 km, relative contrast peak value appears at 2.6-2.8 m. The nearer the target is from the ground, the higher the atmospheric attenuation is to infrared radiation.