Fog is an inevitable weather factor affecting the performance of free space optical communication. Selecting the appropriate laser wavelength can effectively reduce the influence. Three atmospheric visibility of mist, fog, haze were considered in this paper. The transmission characteristic of mid-wave IR wavelength at 3.8 μm was compared with the traditional near-infrared wavelength at 1.55 μm that both belong to atmospheric transmission windows. Using Monte Carlo method, the propagation of a large number of photons along the horizontal direction was numerically simulated. Relationship between the relative light intensity and different receiving surface radius, photon transmittance under different transmission distance and different atmospheric visibility were worked out. The result reveals that the mid-wave IR wavelength at 3.8 μm, the light intensity is more concentrated under the same radius of receiving surface, and photon transmission is higher under the same propagation distance and less affected by atmospheric visibility. The wavelength at 3.8 μm is more suitable for horizontal link of free space optical communication in the fog than 1.55 μm.