As a two-dimensional equivalent mass-spring model based on a rectangular or cylindrical liquid storage tank, the Housner model is used in seismic response analysis of liquid storage tanks with different geometric shapes. However, based on previous studies, this model is not suitable for fluid-structure coupling analysis of complex-shaped nuclear liquid storage structures. To analyze the passive containment cooling system water tanks (PCS tank for short) installed at the top of shield buildings in AP1000 and CAP1400 nuclear power plants, a three-dimensional equivalent mass-spring model is proposed through the introduction of volume correction parameters based on the Housner equivalent mass-spring model of a cylindrical tank. Then, several fluid-structure coupling finite element models of different tank systems, established with the finite element software ADINA, are used for modal analysis. The natural vibration characteristics of liquid sloshing in PCS and corresponding annular water tanks are calculated and analyzed with different box sizes and liquid depths. The calculation results from the integrated finite element and three-dimensional equivalent mass-spring models are compared, and it is found that the proposed three-dimensional equivalent mass-spring model of PCS tank can give reasonable estimated values of dynamic fluid pressure of liquid sloshing under different modes. It is concluded that the proposed model is suitable for dynamic fluid pressure analysis of a complex-shaped PCS tank. This model or method can also be applied to the dynamic fluid pressure analysis of other complex-shaped liquid storage tanks.