Taking a large scale hydropower plant in East Africa as the research object, a 3-D numerical model of the underground cavern is built. Hoek-Brown Failure Criterion was used to obtain mechanical parameters of rock mass, and an equivalent method was applied to simulate system support effect. The excavation and supporting process of the underground cavern group with seven layers in total is simulated to analyze and compare mechanical conditions such as deformation, secondary stress and plastic zone, as well as the general stability. The numerical analysis results indicate that the Hoek-Brown Criterion is practical and reliable and the deformation, stress relaxation and plastic zone depth are all not significant and the whole cavern group is in good stability situation, while only local intersection parts require reinforced supporting. The above mentioned mechanical responses become lower after system supporting, indicating the supporting is effective. The analysis results provide the basis for following design and construction work.