A theoretical model of the side-scattering lidar is established by semi-analytical Monte Carlo method. In the model, the side-scattering mechanism of the laser beam on the atmospheric aerosol is studied under multiple-scattering conditions. Through the theoretical simulation of the model, the spatial distribution of the side-scattered photons as well as the side-scattering intensity and the scattering coefficient curves are obtained. Comparing the scattering coefficient curves obtained by experiments and simulations under visibility conditions of 5.7 km, 8 km and 10.2 km, it is found that with the average scattering factor and the asymmetric factor (μs, g) at (0.7, 0.65), (0.45, 0.45) and (0.2, 0.35), the fit goodness is better than 95%.