The effective seismic load is expanded into the initial elastic modal vector of the structure as a combination of modal loads. Taking the Koyna dam as an example, a plastic damage concrete model and nonlinear time-history analysis method are used to calculate the nonlinear response of the dam caused by various modal loads. Subjected to the Koyna, El Centro and artificial ground motion with apeak value of earthquake acceleration 0. 5g respectively, the displacement responses of dam crest caused by the first 5 modal loads are provided. The results show that the nonlinear displacement response is still dominant by the same mode as modal load, and the weak coupling of modal response can be ignored for the calculation of nonlinear seismic response of gravity dams.