The shield building of AP1000 was designed to protect the steel containment vessel of thenuclear reactor. Therefore, the safety and integrity must be ensured during the plant life inany conditions such as an earthquake. The aim of this paper is to study the effect of waterin the water tank on the response of the AP1000 shield building when subjected to threedimensionalseismic ground acceleration. The smoothed particle hydrodynamics method(SPH) and finite element method (FEM) coupling method is used to numerically simulatethe fluid and structure interaction (FSI) between water in the water tank and the AP1000shield building. Then the grid convergence of FEM and SPH for the AP1000 shield building isanalyzed. Next the modal analysis of the AP1000 shield building with various water levels(WLs) in the water tank is taken. Meanwhile, the pressure due to sloshing and oscillation ofthe water in the gravity drain water tank is studied. The influences of the height of water inthe water tank on the time history of acceleration of the AP1000 shield building are discussed,as well as the distributions of amplification, acceleration, displacement, andstresses of the AP1000 shield building. Research on the relationship between the WLs in thewater tank and the response spectrums of the structure are also taken. The results showthat the high WL in the water tank can limit the vibration of the AP1000 shield building andcan more efficiently dissipate the kinetic energy of the AP1000 shield building by fluidstructureinteraction.