Abstract: The structure with long natural vibration period, such as the large-scaled gravity drain tank on the top of AP1000 containment, is vulnerable to long period ground motion. Particularly, the sloshing phenomena in a low frequency range are of great concern. For this kind of nonlinear problem, numerical simulations are complex and possess certain limitations, therefore, we established the experimental models to measure the impact force of the top cover in different positions with various fill depths. The experiments were carried out on the shaking table and the three cycle sine which has the same frequency with the sloshing water was selected as the input excitation. In addition, by comparing with the calculation method proposed by Lu Daogang, the feasibility and accuracy of this calculation method was further verified. At the same time, we found that the calculation results of small amplitude sloshing were relatively small and the results of large amplitude sloshing was relatively large. Hence, the calculation method is considered to be certain conservative.