Abstract: With improved large mass method and displacement method respectively, the total response and quasi-static components of the internal forces in structures under seismic excitations are obtained directly, and then the dynamic components can be separated by subtracting quasi-static components from total components. This technique for quasi-static/dynamic components presented in the paper can yield results that are identical to those of theoretical method. Based on this technique, the dynamic response analysis of a bridge subjected to non-uniform seismic excitations has been carried out, and the quasi-static/dynamic components of internal forces are separated. It reveals that the quasi-static components depend on the relative displacements between the pier foundations. The seismic responses of structure members can be magnified or decreased irregularly by the resonance or counteraction of the quasi-static components and dynamic components.