Abstract: The water in the containment tank of a nuclear power plant (NPP) sloshes when the structure is subjected to earthquake action. The dynamic response of the containment is affected by the fluid-structure interaction effect. The simplified additional mass method which does not consider the water sloshing and the smoothed particle hydrodynamics (SPH) method which considers the water sloshing are used to study the dynamic response of the containment of NPPs. The dynamic response including the base shear, displacement and acceleration under seven cases of various water levels is obtained when the structure is subjected to the commonly used Northridge earthquake wave and the Shanghai synthetic seismic wave. The comparison of the simulation results of the two methods shows that the dynamic response obtained by the SPH method is smaller than that by the simplified additional mass method. It indicates that the water sloshing in the tank has a damping effect on the overall dynamic response of the structure. The dynamic response at various water levels is analyzed by the SPH method. The results show that when the water level is 4.0 m, the dynamic response of the containment is relatively smaller. Therefore, 0.4 m is a reasonable water storage level of the water tank.