Taiwan is located at the junction area between Philippine Sea Plate and Eurasian Plate. The rock mass is extensively disrupted and abundant in groundwater. Accordingly, the geologic hazards, such as landslides and debris flows, frequently occur, especially during the heavy rainfall. Several tunnel slopes of the SouthLink Line of Taiwan Railway are located under the unfavorable conditions of shallow overburden and water leakage. Therefore, the slope stability will significantly affect the railway operation safety. This study applies the concept of the regional groundwater management to control the short and longterm effects of rainfall and groundwater on the slope stability. We conduct numerical simulations on different drainage patterns so as to increase the slope stability and decrease the hazard risks. Our preliminary results and experience appear to be useful for similar tunnel engineering and regional groundwater management in mountain areas.