The decision of underground power plant axis direction is vital for previous investigations of a pumped-storage power station. This paper took Zhirui Power Station as the project background， where the surrounding rock lithology of the underground powerhouse was relatively simple with lava breccias， the structure of rock mass is predominantly mosaic to sub-massive， with mainly Grade IV and V structural planes， well-developed fissures and poor integrity. The optimal selection of underground power plant axis direction under complicated fractured rock mass environment was systematically studied. The research comprehensively analyzed the stability of surrounding rock under the combination of different structure planes cutting by using Unwedge program on the base of block theory and value principle of the maximum value of the minimum safety factor. The results showed that the axis direction of underground power plant in study area should be NE50°， demonstrating that block theory had practical significance in optimizing the axis direction of underground power plant under complicated fractured rock mass environment in the low-medium stress area.