Abstract: Simplified mechanical model is important for seismic design of large storage tanks. In available studies, the simplified models for large storage tanks mostly ignored the influence of the dome. In this paper, a simplified mechanical model considering the effect of the dome was derived and numerical calculations were carried out for a concrete storage tank of 300,000 m³, and the truncation position of the hydrodynamic pressure with guaranteed accuracy was given, and the model was compared with the Housner’s model to confirm the accuracy of the model. The effect of higher mode was investigated using the mode superposition method, and the dynamic response of the storage tank structure was compared between the simplified mechanical models considering and ignoring the effect of the dome. The results show that to ensure the accuracy of the calculation, the hydrodynamic pressure should be truncated at least in the third term, and the shape function should use the second order cantilever beam mode, as well as superimpose at least one order mode. Ignoring the dome effect will underestimate the base shear and overturning moment of the system.