Abstract: This paper takes Pengkeng landslide as a case study on the basis of the theory of unsaturated soil mechanics. The finite element method is used to calculate and analyze the slope seepage, deformation properties and stability under rainfall infiltration. The calculation results are utilized to study dynamic response mechanism of landslide due to rainfall and the relationship between safety factor and displacement under different real working conditions. Results show that the loose structure of the slip mass has provided a convenient condition for rainwater infiltrated. Thus it changes the spatial distribution of matrix suction and weakens the rock-soil mass parameters. The ground water level is risen under sustained rainfall. Deformation instability occurs in the zone of slope toe, then pulls the slope rear edge and causes the tensile fissures, which forms partial landslide in unsaturated area during rainfall period. In the initial stage of the heavy rainfall, the safety factor of the slope drops rapidly, which easily causes the landslide. The safety coefficient reduces further due to the continued infiltration of rainwater. Landslide occurrence after rainfall should be paid more attention. Safety factor is affected and vibrated with water in different parts of the slope. Finally, the concept of risk factor is proposed to reflect the harmful degree of slope under groundwater level and rainfall condition variation. The relationship between the risk factor and incremental displacement is established to describe the changes of surface displacement of slope measure point at different time periods. The above research and method will be useful and meaningful as reference for the management and early warning of the similar projects.