Based on the distribution characteristic of the vapor and CO2 in the atmosphere, and the traditional calculation method of atmospheric absorption transmittance, the calculation method of atmospheric infrared absorption transmittance is analyzed. The calculation model, calculation steps are provided. Moreover, taking the CO2 as anexample, the CO2 atmospheric absorption transmittance in 2.7 μm is calculated. By simulating, the relations between atmospheric absorption transmittance and target height are obtained.