The infrared atmospheric transmittance is the basis for analyzing the radiating characteristics of infrared target and infrared radiation introspection calculation. But the atmospheric transmittance calculation software as Modtran is hard to be integrated with infrared radiation characteristics measurement system. Based on the empircal formula, a engineering calculation method of infrared atmospheric transmittance suitable for long distance and arbitrary transmission path is presented. The method stratifies the atmosphere based on geographical location, obtains atmospheric model parameters with measured data, achieves a fast-engineering calculation of moderate resolution atmospheric transmittance in the long-range mid-wave infrared. The calculation results show that the average transmittance gotten by this method is close to Modtran and easy to integrate with the real-time system.