ABSTRACT: Guandi Hydropower project is located on the downstream of Yalong River. In the underground caverns area, no regional fault or large-scale weakness zone has been found. The rock masses are fresh and belong mainly to Class II of the Chinese rock mass classification system. However, joints and shear zones are relatively well developed. Then the stability of rock block became a major concern. Stability analysis of determined blocks has been conducted using Block theory. It can be seen that the combination of shear zones with gentle dip angles and big joints with steep angles formed the most common adverse combinations which define removable rock blocks. Considering current supported forces, the removable blocks identified are stable and there is still a certain safety margin.