Abstract：The powerhouse of Laxiwa hydropower station is a lager cavern，where the in-situ stress is 10 to 30 MPa，belonging to high in-situ stress region. Faults f10，f12 and f24 are located in the region A of cavern upstream sidewall. By the analysis of lower hemisphere joint plane projection，it is known that faults f10，f12 and f24 can form key block with volume 2 103 m3. During the course of the cavern excavation，because of action of stress release and adjustment，in period of acceleration deformation，the maximum rate of block deformation reaches 1.22 mm/d，therefore，the stability degree of block is significant to define bolt parameters of region A. Multi-parameter analyses of block unstable mode and stability are conducted by block theory. It is determined that block deformation belongs to the unloading one induced by excavation，but not sliding displacement. Based on block deformation monitoring and stability analysis，only structural reinforcement without special functional reinforcement is carried out，which reduces supporting engineering quantity and improves the construction speed. The results show that，under high in-situ stress，the block large underground cavern is stable，if the block deformation has no influence on the safety of other structures，the unloading deformation is allowable for block without special reinforcement measures.
