Abstract: Ponding-infiltration processes on wide platforms can readily induce local instability of high-fill slopes during heavy rainfall events. This study aims to clarify the slope hydrological processes under intense y rainfall and to reveal their influences on the local instabilities of the high-fill slopes, thereby providing a scientific basis for engineering design and disaster prevention of this kind of slope. The numerical models for coupling surface water and groundwater processes and simulating the stress-strain state of soil were built through COMSOL. A variety of numerical simulations were carried out to analyze the relationship between the ponding-infiltration processes on the wide platform and the local instability of high-fill slope. Rainfall infiltration forms transient saturated zones in the shallow filling body. When the transient saturated zone expands to the vicinity of the filling interface, the flow direction of groundwater changes, causing groundwater to migrate along the filling interface. It can be observed that the groundwater level rises with time, and that groundwater discharges to surface water near the retaining wall. The increase of slope water content diminishes the soil shear strength, associated with an interface effect, which is especially obvious in the saturated area near the retaining wall (with the maximum reduction of 70.3%). Ponding on the wide platform raises the local groundwater level and increases slope deformation, thereby reducing slope stability. It is found that of the value and duration of the instability of the slope are larger and longer compared with no ponding considered. The installation of anti-seepage geotextiles under the wide platforms can improve the stability of the slope by reducing groundwater seepage. Specifically, the spatial continuity of the transient saturated zones is decreased, reducing the hydraulic connectivity of groundwater seepage channels and the groundwater level at the foot of the slope. Ponding-infiltration processes on wide platforms of high-fill slope can rise the groundwater level, strengthen the interaction of surface water and groundwater, increase the pore water pressure of the shallow soil, and aggravate local deformation, which decrease the stability of the slope. The use of anti-seepage geotextiles under the wide platforms can reduce rainfall infiltration and improve the stability of the slope.