A mathematical model is established by the Mie scattering theory and the pulse multipath effect to predict the maximum communication rate based on the horizontal atmospheric visibility. With the method of Monte Carlo simulation, we obtain the scattering functions of the aerosol scattering particles at 800 nm and 1550 nm, respectively, and the pulse broadening curves of the two wavebands in the visibilities of 2-25 km. The influence of the initial pulse width, the pulse position modulation (PPM) element and the horizontal visibility on the communication rate is analyzed. Results show that the communication effect is better in 1550 nm band and the maximum communication rate improves with the increasing visibility. Under the good visibility, the small initial pulse width and PPM element can maximize the communication rate.