The influence of pore water pressure on the dynamic stability of slope under seismic action is studied in this paper with a reservoir dam as research object. The temporal changes of seepage field caused by seismic load are analyzed first, and then the stability of dam slope is investigated by coupling the stress field with the seepage field. Finite element method is used to analyze seepage field and vibration field, while limit equilibrium method is used to analyze slope stability. Results demonstrate that seismic action stimulates the increase of excess pore water pressure, and whether to consider the coupling effect of seismic load and pore pressure change will make the results of stability analysis quite different. When the coupling effect is not considered, the slope is prone to shallow sliding, with flat sliding surface; as a result, the seismic stability of dam slope is overestimated. On the contrary, when the coupling effect is taken into account, the position of slip surface is obviously deeper, which easily leads to deep sliding. The cumulative effect of pore pressure may be the crucial factor of the dynamic instability of two-phase media slopes. In conclusion, pore water pressure under seismic action plays an important role in the analysis of slope dynamic stability.