It wasn＇t discussed the effect from air density, different activity from uranium, thorium and K-40, density of lithology and exploration height in previous model about the relationship between the lithology and the ratio of low energy part of airborne γ- ray Spectra. Based on the idea for establishing hexagonal models to cali- brate airborne gamma – ray spectrometry, a new Monte Carlo model is put forward to resolve the problem of deep penetration in airborne γ – ray spectra simulation. Based on the MC model, air density was 1. 289 ×10 -3 g/m3 by comparing with measuring values; the middle two have no effect on the ratio of low energy spectrum and an exponential formula can revise the effect from exploration height. The ability of the above correction model to discern lithology has been increased by outdoor experiment.