Rainfall infiltration temporarily increases water pressure in the unsaturated area above the groundwater level of a landslide, reduces matrix suction, and weakens the shear strength of the slope. Meanwhile, cracks in the landslide facilitate rapid rainwater infiltration. During rainy season, intense rainfall accumulates and forms a transient water source in areas with more developed cracks in the slope. Continuous recharge results in high pore dynamic water pressure, leading to a rise in the groundwater level of deep landslide. This transient movement of groundwater and the associated additional water load become controlling factors for slope instability during rainy season. In this study we focus on large-scale accumulation landslides. Based on the saturated-unsaturated seepage theory of soil mass, we examined the influence of transient water sources formed by heavy rainfall at the end of cracks in the slope on the seepage field and groundwater level of landslide using finite difference method. We evaluated the adverse effect of transient water supply on the stability of the landslide by using the strength reduction method. The research findings provide a scientific basis for landslide prevention and the design of drainage treatment.