Abstract: Passive ranging an aerocraft using its exhaust plume infrared radiation was studied. Exhaust system of an aircraft or a missile is the main infrared radiant point in 3-5 m wave-band. The infrared radiant characteristic of rectangular nozzle flame was calculated by modeling. The result shows that the infrared radiation intensity color ratio of an exhaust plume is same in different direction. When the infrared radiation transmitted in the atmosphere, it would be attenuated by the absorption and dispersion of the atmosphere. The attenuation was different according to different wavelength. The radiation intensity color ratio of different wavelength would change along with the change of transmission distance. The radiation intensity color ratio contains distance information. The relationship among the color ratio of exhaust plume and its temperature, its transmission distance, and atmosphere attenuation coefficient were deduced. The relationship between color ratio of an exhaust plume and its temperature was calculated. Infrared three color ratio ranging equations were deduced. The three wavelength of 4.5, 4.6, 4.7 mm was used to study passive ranging method. A ranging figure was drawn using the equations. The result shows that the method can be used to range an exhaust plume.