The polarized multiple scattering of multi-layer discrete random media by laser is simulated by the Monte Carlo method. The simulation steps of a fast semi-analytic Monte Carlo method are given based on the properties of the polarization lidar. The free path adjustment is considered when the photon crosses the interface of different media. The scattering angles for different media layers are sampled based on the Mie theory. The relationship between lidar depolarization ratio and the penetration depth of two-layer and three-layer water clouds is studied. The results show that the depolarization ratio increases with the depth of penetration, and the increasing speed of depolarization ratio is also different when laser is incident from one medium to another medium. The influence of effective size, single scattering phase function, extinction on the lidar depolarization is analyzed. The method can be used to retrieve the microphysical and optical characteristics of inhomogeneous clouds or aerosols with the polarization lidar.