Earthquake is one of the main factors that induce slope instability. As a widely used geotechnical retaining structure, the seismic stability of gravity retaining walls remains a popular topic in geotechnical engineering. A semi-analytical horizontal slice method is proposed on the basis of the upper bound limit theory and the pseudodynamic approach to evaluate the active earth pressure of unsaturated filling soil under seismic action effectively. The work rates of unsaturated soil gravity and seismic inertia force with evident nonlinear distribution characteristics were computed to formulate the energy balance equation. The semi-analytical solution of active earth pressure was formulated explicitly according to the equation and then compared with the analytical solution to validate the proposed method. Parametric analyses of the impacts of soil suction and seismic excitations on the earth pressure estimations were also conducted. The findings demonstrate the following: (1) the active earth pressure is overestimated when soil suction is disregarded in the analyses. The suction effect relates not only to the type of filling soil but also to the characteristics of ground motion. (2) The active earth pressure depends significantly on the horizontal and vertical ground motions. The peak earth pressure slightly increases with the soil shear modulus and moves toward the negative direction; as the seismic period increases, the peak earth pressure slightly rises and moves toward the positive direction. (3) The impact of surcharge load on the earth pressure becomes increasingly evident in filling soil with large inclination angles. For filling soil with an inclination angle larger than 100^°, the earth pressure becomes markedly pronounced with large soil-wall friction angles.