To understand and improve the anti-wave impact performance of coastal breakwater, the dynamic behaviors of the breakwater under wave impact were numerically simulated. The wave movement was modeled by using the multi-material ALE method. The interaction between the breakwater structure and fluid was studied by applying the penalty method. The model and approaches were validated by comparing numerical results with experimental data. Based on the above investigation, numerical simulation was performed to explore the distribution of seawall surface wave pressure and the maximum horizontal thrust. The effect of breakwater structure response to wave pressure was also dealt with in this research. The results show that: the wave pressures were in good agreement with experimental results, the wave impact was enhanced by structural dynamic response, and the rear wall withstands the larger wave horizontal thrust. It can provide references for the anti-wave impact design and overtopping design of the coastal Nuclear Power Plant breakwater.