Abstract: Containment external water tank is used for long-term heat removal from the inner containment at beyond design basis accidents for China-designed third-generation nuclear power plant. It is a critical facility to ensure the integrity of the containment and the safety of nuclear power plants. Therefore, it is necessary to analyze the structural strength of the external water tank under the load of the extreme safety ground motion combined with abnormal temperature operating conditions. In this paper, the finite element model of the outer containment with external water tank was established, grid sensitivity analysis was conducted, and the structural vibration characteristics were analyzed through the modal analysis. The time history analysis method was used, and the structure was under the load of temperature and earthquake. The fluid-structure interaction method was used to simulate the interaction between the fluid and the structure, analyze the seismic response of the external tank structure and fluid oscillation. The result shows that the fluid oscillates strongly on the water surface at the lower position of the external tank, and the absolute values of the maximum tensile strain and the maximum compressive strain of the external water tank in three cases of the EL Centro seismic, the synthetic seismic and the long-period seismic are less than the allowable values of the C60 concrete.