Taking into consideration the nonlinear characteristics of the initial soil moisture content with depth, the boundary conditions of rainfall infiltration, and the dynamic development of saturated and unsaturated zones, a modification has been made of the Green-Ampt (GA) model, followed by a study on the practicality and parameter influence of the modified model based on a parameter calculation and analysis. The research results indicate that the improved GA model divides the rainfall infiltration process into three stages, thus deriving corresponding time boundary conditions, as well as calculating the duration of rainfall and the depth of the wetting front and saturation zone in each stage. When the rainfall intensity is low, the rainfall infiltration process is divided into three stages, but as the rainfall intensity increases, the rainfall infiltration process can be simplified into two stages. When a rainfall intensity is relatively small, the rainfall infiltration process can be divided into three stages, while with the increase of rainfall intensity, the rainfall infiltration process can be simplified to 2 stages. The initial moisture content distribution mode has exerted a significant influence on the soil infiltration time boundary condition, wetting front and saturation zone depth. As the duration of rainfall increases, the development of the wetting front depth in the exponential distribution pattern is faster than that in the fixed value distribution pattern. In addition, in the early stage of rainfall infiltration, the initial moisture content distribution pattern has a significant impact on the parameter ξ, but the impact on ξ can gradually be ignored with an increase of rainfall duration.