Abstract: Weak interlayer and rainfall have great effects on slope stability. The weak interlayer may change the failure mechanism and reduce the slope stability. The rainfall will make the slope more instable and may even induce weak interlayer formed in the slope. However, simple and rational evaluation methods for such a slope stability problem have not been proposed so far. In this paper, based on the upper bound limit analysis method, a new failure mechanism consisting of rigid blocks undergoing impending rotational or translational movements is proposed to study the slope stability under rainfall condition. Rainfall-induced change of pore water pressure in slopes is analyzed by means of the transient seepage finite element analysis. The saturation line and the soil strength after rainfall are then considered and incorporated into the limit analysis. The proposed rotation-translation combined mechanism is finally verified by comparisons with the elasto-plasticity finite element method. The comparisons show that the proposed mechanism improves the efficiency of the previously-developed rigid block mechanism to account for the presence of weak interlayer in the slopes and can be used as a simple evaluation method for the engineering design.