Abstract: Atmosphere transmittance, which is an important factor influencing the operating range of infrared system and couldn-t be described precisely because of its complex changing with the atmospheric environment, wavelength and range etc, becomes one of difficulties for calculating the operating range. The impact of atmospheric transmittance on the effect distance of the infrared system was studied. An infrared detection range model based on discrete spectral transmittance was proposed. The atmospheric transmittance of the 1 cm-1 wave number segments was calculated by the MODTRAN software and was used to calculate transmitting infrared radiation power in the 1 cm-1 wave number segments. The calculated data was used to calculate the operating range of the infrared image system, instead of using a constant or average atmospheric transmittance in traditional method. The presented model is applied to the calculation of Tomahawk cruise missiles. Results indicate that it has a high accuracy and feasibility.