According to the minimum resolvable contrast criterion, the method for calculating the maximum detection range of an airborne infrared imaging system to a target in a background environment is studied. The infrared radiation characteristics of the infrared target in two wavebands of 3 to 5 μm and 8 to 14 μm and the influence of background radiation and atmospheric transmittance etc on the detection range are analyzed. The derivative formula of the detection range is given. Finally, the atmospheric transmittance is simulated with the MODTRAN. The atmospheric spectral transmission data with a wave number interval of 1 cm_-1 is obtained. The irradiance on the detector in each mini wavelength region is calculated. By taking the relationship between the atmospheric transmittance and the distance, the replacement of atmospheric transmittance by constant or fitting function in the traditional algorithm is canceled. Taking the cruise missile as an example, the detection range of an infrared system is calculated numerically. The simulation result shows that this calculation method is feasible.