Based on the dynamic elasto-plastic methods, the seismic response and failure mechanism of breakwater for nuclear power plant under earthquakes is analyzed. The failure mechanism is studied through the research of the shear strain, the permanent deformation as well as the displacement response during earthquakes considering the seismic wave input of one-direction and two-direction. The main concerned aspects are the maximum displacement response and the permanent deformation of wave barrier, and the results show that the vertical seismic input has obvious impact on the slope stability. At the same time, the influences of parameter changes of the length of slope protection, ground motion and remarkable frequency on failure modes of breakwater are systematically studied. The results are valuable for the design of projects, the standard of safety assessment and the choice of the anti-seismic measures.