In this paper, the force against rigid retaining walls from backfill soil under static loads and vibration loads is analyzed within three cases. The first case is an ordinary retaining wall without expanded polystyrene (EPS) geofoam buffer. In the second and the third case, a layer of vertical EPS buffer with different density and elastic modulus is placed between a rigid retaining wall and backfill soil. Numerical simulation results show that the force against the same retaining wall in the treated cases is less than that in the untreated case, under both static loads and vibration loads. Moreover, the compression of different EPS buffer is studied. Under vibration excitation, when the density and elastic modulus of EPS buffer decreases, its compression increases and more wall force is mitigated. Simulation results accord with the physical shaking table test data. Numerical results and physical test demonstrate that EPS geofoam seismic buffers hold great promise to reduce loads against rigid retaining wall structures, especially earthquake-induced dynamic loads.