Based on the research of empirical rainfall threshold and numerical analysis, the landslide early warning rainfall threshold was calculated with advanced model of FEM code Plaxis, considering the rainfall infiltration and the characteristics of unsaturated soil. The influence of rainfall distribution mode was firstly studied. The results show that separating rainfall into two phases is better than one averaged phase in slope stability calculation. Then, the antecedent rainfall is redefined with explicit physical significance as the rainfall causes the change of matric suction of the unsaturated soil within the slope surface and the potential slip surface. And the antecedent rainfall duration is directly proportional to maximum depth of potential slip surface and inversely proportional to the saturated permeability coefficient of the soil. The two parameters in the expression of the duration are investigated and the suggested values are given. Lastly, the amount of antecedent rainfall is introduced to the I-D curve as the third coordinate axis to develop A-I-D rainfall threshold surface. A case study of slope on bed rock is carried out to demonstrate the process for establishing this space curve, which is compared with the test data. This verification proves the reliability of the method proposed.