Taking the gravity dam of the lower reservoir of a pumped storage power station as an example, according to the basic theory of the mode decomposition response spectrum method, the natural vibration characteristics and seismic response of the dam were calculated using the cantilever beam method. The dynamic responses such as natural vibration frequency, modal shape, acceleration, dynamic stress and anti-sliding stability were obtained, and the results were compared with those calculated using the finite element method. The results show that the relative error between the dam fundamental frequency values calculated by the cantilever beam method and the finite element method is 4.24%. Under the normal water level and design earthquake conditions, the distribution law of the dam acceleration along the river calculated by both methods is basically the same, and the relative error of the acceleration extreme value is 2.13%. The distribution patterns and extreme values of the vertical normal stress of dam after static and dynamic superposition obtained from both methods are similar. The safety factor of anti-sliding stability along the foundation surface calculated by the cantilever beam method is greater than that calculated by the finite element method. The dynamic response calculation of the cantilever beam method is simple, and the results are reasonable and reliable, which is of practical value for the rapid seismic safety analysis of gravity dams.