In order to develop a prediciion method for the occurring time of a slope failure based on the critical rainfall, the following studies were carried out. Conditions which has influence on the critical rainfall were discussed. Then, the change of pressure head in natural condition were investigated. Finally, the pore pressure distribution in a slope and the stability of slope were analysed numerically.Consequently, the following results are obtained:(1) The critical rainfall curve which is the relation between rainfall per a week and rainfall per a day of the failed slopes can be obtained numerically.(2) The seepage characteristic and the thickness of permeable layer have a great influence on the total rainfall till the slope failure.(3) The quantity of successive rainfall for the slope failure can be accurately estimated by numerical analysis if the initial pore pressure distribution in the slope is known.