Excess pore water pressure has a significant effect on the occurrence of landslides under earthquake shaking. For seismic displacement calculation using the sliding block method, the prevailing idea about the generation of excess pore water pressure is mostly in fully saturated soils. However, the existence of shallow landslides with partially saturated soils has been demonstrated. In the present study, an empirical partially saturated excess pore water pressure model is applied to the nonlinear analysis of flexible sliding block method for sandy slope, and the degree of saturation ranges from 10% to 100%. The dynamic shear modulus and damping ratio are considered, and the effect of degree of saturation variation on yield acceleration and slope displacement is investigated in this study. Detailed explanations are made to the effects of suction stress, soil mass, and excess pore water pressure on the seismic response of slopes at different degrees of saturation. The results clearly demonstrate that the variation of degree of saturation is of significance, the excess pore pressure ratio increases with saturation, showing an exponential function relationship, and the change of yield acceleration and displacement with saturation is complex, which can be divided into two cases: suction stress dominant and excess pore water pressure ratio dominant.