Abstract: The conventional methods for slope stability analysis are mainly based on the theory of saturated soils and the stable seepage situation and difficult to reflect the influence of rainfall and water level drawdown on slope stability. Actually quite a number of slope instabilities are caused by transient unsaturated seepage. Based on the effective principle of unsaturated soils by Bishop and the theory of two-stress-state-variable mechanism of the shear strength by Fredlund, the water pressure at any time is introduced into the balance equation as the volume force and the effective stress fields satisfying the conditions of equilibrium, stress boundary, yielding and stress discontinuity are constructed. By using the idea of nonlinear programming, a lower bound analysis model for the slope stability subjected to transient unsaturated seepage is established, and the corresponding program is compiled. Through numerical examples of impoundment drawdown and rainfall infiltration analysis, the correctness of the proposed method is verified, and a further understanding about the law of slope instability subjected to transient unsaturated seepage is obtained.