A lot of lives, infrastructures and heritages were lost due to slope failure. Therefore, it is necessary to construct a prevention system in order to predict the probability of slope failure due to heavy rainfall. We have proposed the research strategy of the prevention system, which consists of the laboratory soil tests on disturbed and undisturbed samples, the numerical simulations, seepage and failure tests, the filed measurements of suction, the temperature and rainfall, and the some in-situ tests for identification of geological and geotechnical characteristics of slope. In this paper, numerical simulation for the evaluation of slope instability due to rainfall will be presented. Furthermore, experiments are also conducted in this paper in order to validate the simulation models. The results of this paper have shown that the stability of the slope during rainfall can be well evaluated by using our method.