The lidar returns exist a depolarization signal in water clouds due to the occurrence of multiple scattering when detecting water clouds. There exist some relationships between depolarization ratio and the microphysics characteristics of water clouds, and they can be used to retrieve the microphysics characteristics of water clouds. However, the relationships must be studied and explored by theoretical simulations. An important method used to simulate the lidar depolarization is Monte Carlo method. Two Monte Carlo simulation programs based on different ideas are given. The first approach is based on tracking the meridian plane described by the photon direction vector and the Z-aixs after each scattering step, and the polarization relative to the new meridian plane is described by the Stokes vector. The second approach is based on the following idea, the polarization state of the photon is described by a triplet of unit vectors rotated by an azimuth and scattering angle after each scattering step so as to track the polarization state change of the photon. A detailed simulation process is given for each approach, and the depolarization ratio for a type of water clouds is calculated using these two approaches. The simulation results show that the results calculated by the two Monte Carlo methods are in good agreement.