During the simulation of rainwater infiltration, the conventional rainfall boundary cannot accurately reflect the influence of the variation of ponding water depth on the calculation of actual rainwater infiltration rate. To address this issue, the conventional rainfall boundary is improved by incorporating the diffusion wave approximation equation, allowing for the coupling of ponding water depth variations with actual infiltration rate during heavy rainfall events. Additionally, the accuracy of the improved boundary is validated through two classical experimental cases. The improved boundary is subsequently applied to the simulation of an actual engineering scenario. The findings indicate that the improved rainfall boundary is capable of achieving real-time dynamic transition between flux and pressure head boundaries. When the rainfall boundary functions as the pressure head boundary, the theoretical maximum ponding water depth calculated by the improved boundary is located at the slope toe. Conversely, when the rainfall boundary serves as the flux boundary during the final stage of rainfall, the ponding water depth becomes negligible, and the theoretical maximum ponding water depth is located at the intersection of the flat and steep slope surface.