Abstract: The seismic excitation remains the main factor in triggering the slope failures that present three-dimensional (3D) features universally. The slopes combined with anti-slide piles are the effective reinforcement techniques in improving the slope stability and are widely employed in slope failure preventions. Based on the upper bound limit analysis theorem and the pseudo-dynamic approach, the plasticity deformation theory is extended to unsaturated soils, and a modified horizontal slice method is proposed to calculate the work rates of unsaturated soil gravity, seismic inertial forces and pile lateral resistance forces as well as the energy dissipation rate of apparent cohesion that are all characterized with nonlinear distribution features. The semi-analytical method is validated through comparisons with the analytical solutions. A series of parametric analyses are conducted regarding the seismic responses of unsaturated soil slopes. The improvements in seismic slope stability due to soil suction are studied systematically. The results show that the slope safety is underestimated by 15% ~ 30% when the soil suction is not considered in the analyses. The suction effects depend on the seismic excitations and are enhanced significantly when the seismic wave approaches the peak. The horizontal acceleration coefficient and soil shear modulus both have a great influence on the slope stability. The critical slip surface becomes much shallower under excitation conditions.