Atmospheric transmittance is an important factor affecting the operating distance of an infrared imaging system. In the process of atmospheric transmission, the infrared radiation may be affected by the selective absorption of atmospheric molecules, scattering and complicated weather conditions. This makes the atmospheric transmittance become a complex parameter. A spectral bisection method is used to calculate the atmospheric transmittance. Firstly, the MODTRAN software is used to calculate the atmospheric transmittance of each spectral line at a certain distance and establish a database. Then, the spectral region is divided at a certain interval and the atmospheric transmittance in each mini spectral region is calculated by calling the database. Finally, the calculated result is put into the model to calculate the operating distance of the infrared system. Compared with the calculation method which uses constant or average atmospheric transmittance, the calculation accuracy of the proposed method is improved. The effects of detecting height, humidity and fog on MRTD and operating distance of an infrared imaging system are calculated and analyzed.