Abstract：Based on the finite element method of saturated and unsaturated rainfall infiltrations and the strength theory of unsaturated soils，a strength reduction method of slope stability analysis was presented. The method could consider the influence of strength variation of unsaturated zone and seepage load in saturated zone. It was applied to the deformation calculation and stability evaluation for the engineering slope K261 of Yuanmo highway under the influence of precipitation. It showed that the range of unsaturated zone would decrease；the pressure water head would increase and the transient saturated zone would appear because of the strong rainfall. The factor of safety of engineering slope would obviously decrease due to the unsaturated zone strength decreasing and the load changing of saturated zone seepage. The engineering slope would be unsteady under the extreme condition of constant rainfall accompanied with seismic loading. Preventing rainfall infiltration from slope and draining of slope could improve the stability of engineering slope effectively. At the same time，some resisting slip piles are suggested to for the 6-grade engineering slope.
