In order to study the transmission characteristics of infrared radiation in fog under multiple scattering effect, the extinction parameters and multi-particle scattering phase functions of 1.064 μm, 3.8 μm and 10.6 μm laser in advection fog and radiation fog were calculated based on Mie theory. A radiative transfer model was established by Monte Carlo method. The effects of the size of the receiving screen, the visibility and the transmission distance on the transmittance were analyzed, and the results were compared with Lambert-Beer law. The results show that when the visibility and transmission distance are both 1 km, the number of particles on the receiving screen of 1.064 μm laser in advection fog increases significantly with the increase of receiving screen side length. The attenuation of laser in radiation fog is less than that in advection fog under the same visibility and transmission distance. 10.6 μm laser has good transport properties in advection fog. The stronger the scattering ability and the higher the forward scattering probability, the more obvious the contribution of multiple scattering to the transmittance. The attenuation characteristics of laser transmission in fog are related to extinction coefficient and scattering coefficient. There is an optimal combination of visibility and transmission distance which maximizes the contribution of multiple scattering to the transmittance.