An infrared atmospheric transmission model is established based on photon tracing，and the theoretical equation for pass loss has also been derived. Through comparing pass losses calculated by Lambert-Beer law and Monte- Carlo method we know that the scattering energy received by the receiver cannot be ignored. The transmission characteristics of near infrared, mid- infrared and far infrared under different visibilities and communication ranges are achieved. The results reveal that the pass loss of 10.6 μm is the minimum under the radiation fog. While under the advection fog, 0.85 μm has the minimum pass loss when the visibility is below 200 m, and 10.6 μm has the minimum pass loss when the visibility is over 200 m.