Scattering is the fundamental issue in the field of underwater optics, meanwhile polarization state is also a key feature in underwater laser communications, laser imaging, and underwater quantum optical communication. Aiming at this problem, a software package Geant4 is applied in the simulation, furthermore, an algorithm based on the basic Henyey-Greenstein phase functions and typical polarization Monte Carlo model is proposed, the algorithm is used to simulate the polarization state when the laser is propagating underwater at different concentrations of water or transmission length. At the same time, the experiment is tested in a sink whose length is 10m and the polarization state detector is a laser power meter with accuracy up to 10nW. In this paper, the change of polarization state is expressed by the depolarization ratio. The result of numerical simulation and experimental show the variation of polarization state with concentration and distance respectively, the simulation result of Geant4 is more ideal, due to different kinds of external factors the experimental results are roughly, however, the overall change trend of the two is consistent.