Abstract: In the design of large liquid-storage spherical tanks, the added mass method based on Housner's theory suffers from insufficient accuracy, while the acoustic-structural coupling method requires substantial computational resources for seismic analysis. Therefore, this paper proposed a method for establishing an equivalent acoustic-structural coupling model. Taking the passive emergency core cooling vessel as the research object, both an acoustic-structural coupling model and an equivalent acoustic-structural coupling model were developed. In the equivalent model, the water domain was modeled using solid elements, and the low-order modal natural frequencies of the model were adjusted by modifying the elastic modulus of the water domain. The results were then compared with those of the acoustic-structural coupling model. The maximum deviation in the first five modal frequencies compared to the acoustic-structural coupling model was only 1.58%, confirming the accuracy and reliability of the equivalent model. Finally, the seismic analysis of the spherical tank was conducted using the equivalent model, demonstrating that the equivalent model has good convergence.