Calculation of atmosphere transmittance is an important prerequisite for effective infrared characterization of airborne targets. In this paper, for the law of infrared absorption transmittance of lower atmosphere changing with altitude, a numerical simulation model is established. Based on the fact that water vapor and CO₂ absorptive capacity in the lower atmosphere are affected by altitude, the important effect of atmospheric density change with altitude is added, and the absorption transmittance of the lower atmosphere is analyzed comprehensively for both horizontal and inclined path. The simulation experiments are conducted to compare the results of the numerical simulation model and MODTRAN5, and the reliability and validity of the model is verified