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. Compared with two-dimensional method, three-dimensional method can reflect the condition of slope more closely to the fact. In this paper, based on the upper bound limit analysis method, a new three-dimensional rotation-translation combined mechanism is proposed to study the stability of slope with weak interlayer under rainfall condition. The mechanism consists of the cone translational mechanism and the spiral cone rotational mechanism. And this combined mechanism can be expended longitudinally through the plane insert, and hence the method could be used to analyze the slope stability with different lengths. Pore water pressures in slopes are analyzed by means of the two-dimensional seepage finite element analysis and considered as external force in the limit analysis. The proposed mechanism is finally verified by comparisons with the SSRFEM and two-dimensional limit analysis method. The comparisons show that the proposed mechanism can be used as a simple evaluation method for the engineering design.