The polarization lidar returns exist a depolarization signal due to the occurrence of multiple scattering when detecting the microphysics characteristics of liquid water clouds by using lidar. The depolarization information can be used to quantitatively retrieve the clouds characteristics. The semi-analytic Monte Carlo simulation was used to study the backscattering depolarization ratio of polarization lidar with the wavelength of 0.532 ?滋m to the height of clouds base, size of the clouds droplets and the extinction coefficient. From the results, if the receiving angle of field remains constant, the depolarization ratio increases with the increase of the distance between the lidar and the clouds and the increase of the extinction coefficients. The impacts of multiple scattering increase with the increase of the receiver field of view angles, so a bigger receiver field of the angle will generate a bigger depolarization ratio. The depolarization ratio is bigger for the clouds with a smaller clouds droplets when the penetration depth is small, but it will be bigger for the clouds with a bigger clouds droplets when the penetration depth is more deeply.