Tseng Wen Reservoir, which has the largest capacity in Taiwan, is the main water source in south Taiwan. Operating since 1974, the reservoir’s storage capacity decreases due to sediment deposition. Due to the lack of silt facilities in the bottom of the dam, a new sediment sluicing tunnel is planned under the left side of the dam so as to extend the service life of the reservoir. The tunnel outlet river does not have sufficient area to provide energy dissipation, therefore an underground cavern with a size of L190 m×D23 m×H46 m is needed. In this study, threedimensional numerical model analysis is used to provide rock mass parameter assumptions and to review the designed support system so as to ensure the safety of the underground excavation. The paper also carries instructions for key issues encountered in the region, which includes rock mass bearing capacity at the top of the tunnel arch, the bifurcation region treatment measures of the cavern, countermeasures to cope with the asymmetrical pressure of the slope, etc.. 