Abstract: Based on the Richards's equation which describes the behaviour of capillary conduction of liquids in porous mediums, Iverson derived the equation which expresses the response of the pore-water pressure at any depth in the slope to rainfall infiltration in 2000. The formula which calculates the factor of safety of the shallow landslide trigged by rainfall infiltration was derived on the basis above in this paper. The formula takes account of the variation of the gravity and the degree of saturation in the unsaturated area in the slope under the rainfall infiltration. The results of the case study show that: the factor of safety of the shallow landslide gradually decreases with the rainfall infiltration and reaches the minimum value in a short time after the cease of rainfall infiltration. After that the factor of safety gradually increases with the rate which was lower than the one of decreasing. The variation trends above at the different initial vertical groundwater depths are similar; however, the factor of safety decreases with the increase of initial vertical groundwater depth. The duration of rainfall infiltration and rainfall infiltration rate both have the marked impact on the factor of safety of shallow landslide. The impact show that the larger of the rainfall infiltration rate or longer of the duration of rainfall infiltration, the larger decreasing amount of factor of safety is. However, the impact caused by the rainfall infiltration rate was controlled by the vertical permeability coefficient of the landslide.