A dynamic three-phase coupled analysis is validated through numerical simulations of seismic behavior of a damaged fill slope during an earthquake. The equations governing the dynamic deformation of unsaturated soil are derived here based on porous media theory and constitutive models. The skeleton stress-strain constitutive relation is a simplified elasto-plastic constitutive model based on the non-associated flow rule and nonlinear kinematic hardening rule. The soil water characteristic model is a hysteresis model to reproduce the wetting process during cyclic loading. An actual fill slope which was partially collapsed and flowed during the 2003 earthquake is examined. The physical and mechanical properties are investigated with in-situ and laboratory tests. The numerical results showed the reduction of skeleton stress in the unsaturated fill as well as liquefaction in the saturated fill during the earthquake.