Stable initial groundwater exists in most shallow slopes in mountainous and hilly areas of Zhejiang province, which leads to complex distribution of soil water content and difficulty of rainfall infiltration process and slope stability analysis. Considering the characteristics of unsaturated soil, the exponential distribution model of water content of slope under initial groundwater was determined. The rainfall condition was divided into weak and heavy rainfall according to the relationship between rainfall intensity and soil saturated permeability coefficient. On the basis of the Green-Ampt infiltration model and water balance principle, the analytical solution of governing equations for rainfall infiltration of shallow slope with groundwater was derived for the two conditions. Combined with the shallow infinite slope model, the stability of shallow slope under rainfall was analyzed considering the influence of initial groundwater. The modified model could be approximately degenerated into the traditional model when the initial water content was uniformly distributed. When the initial water content was in exponential distribution, the effectiveness of the modified model was verified through numerical simulation. Results showed that shallow slope failure induced by rainfall occurred near the bedrock surface when groundwater existed in the soil layer over the bedrock in shallow slope, and the failure time was prior to that predicted by the traditional model. Under weak rainfall conditions, the failure time was nearly exponentially shortened with the increase in rainfall intensity. Under heavy rainfall conditions, the failure time tended to be stable when the rainfall intensity was increased, on the condition that the promoting effect of slope surface runoff on infiltration was not considered.