As the color ratio of radiation intensity of a target at different wavelengths will change along with the change of transmission distance, the color ratio of radiation intensity contains distance information. According the principle mentioned above, this paper proposed a scheme based on colorimetric method to range the target using its own infrared radiation. The relationship between the color ratio of radiation intensity for the target and its temperature, radiation transmission distance, atmosphere attenuation coefficient was deduced. By measuring the color ratio of radiation intensity at different wavelengths, the target was ranged when the atmosphere attenuation was certain and the infrared ranging equations of three-color ratio were deduced. According to a calculation model of radiation atmosphere transmission attenuation, three wavelengths of 8.5,9.0,9.5 μm were selected as color-ratio wavelengths, a 300 K blackbody 2 km away was used as a ranging target, and a color-ratio ranging figure was drawn. Analysis shows that the temperature and the distance of the target gotten from the figure are coincident with those of the target. Furthermore, it needs only once measurement to get the distance and temperature of the target by using this method.