Rainfall-induced shallow landslide is a type of commonly occurring geohazards. To explore the evolution of the slope stability during a rainfall event, a conceptual model of rainfall-induced shallow landslide with considering the effect of transient pore water pressure is developed based on the Green-Ampt infiltration model, and the relationship between the factor of safety and precipitation duration for the slope with or without ground water is developed. The results show that for the slope with underground water, the pore water pressure in the slip zone drastically increases once after the wetting front touches the groundwater table; for the slope without ground water, the slip zone become inundated in the water and the soil strength decrease quickly. Based on these results, and according to whether the shallow landslide is completely saturated or not during rainfall, a concept of critical saturating time is proposed, with considering the case that the initial rainfall intensity is less than the soil’s infiltration ability. This critical time can be used as a parameter for early warning the shallow landslide.