Abstract: The rubble mound breakwater is considered as a discrete medium structure according to the fact that the blocks in the section of the breakwater are generated randomly, and that the porosity region among the blocks of the breakwater is meshed into grids. Based on the N-S equation for the planar viscous incompressible fluid and the VOF method for the disposal of the free surface, the finite element method model of fluid field inside of, in the front of and in the rear part of the breakwater under the wave effect is established separately, the numerical simulation of the wave motion inside the breakwater is carried out. Numerical results agree well with the wave tank test results in trend. Numerical results of the breakwater with different porosities and different block radius show that the fluid field inside of the breakwater is affected by many parameters, that the transmission coefficient increases with higher porosity and longer wavelength, and that the forces on the blocks decrease with the increase of porosity and average radius.