Abstract: Seismic analysis of gravity dams with five different heights in time domains was performed on the fluid-structure coupling model. Compared to fluid-structure coupling results, Westergaard solutions underestimate the hydrodynamic pressures in the upper part of the dam body, and overestimate in the lower part of the body. The overestimation is more significance with the increase of dam heights. The position where the maximal dynamic water pressure appears in the fluid-structure coupling model increases in height with the increase of the height of dams. On this basis, Westergaard equation was modified taking into account the influence of dam height, elasticity and reservoir bottom condition. The modified Westergaard formula solutions are quite coincident with the results of fluid-structure coupling models and other existing researches.