Abstract: The typical failure modes of blocks for the Three Gorges Project underground powerhouse are presented based on analysis of geological investigation in the site and block theory under the condition of only considering their gravity.The characteristics of secondary stress field and deformation of different position blocks under the condition of different nature stress field are explored by 3-D numerical simulation and deformation monitor,the characters of localizing deformation of blocks and influence of stress of surrounding rock mass on their stability are revealed,and their deformation evolution modes are presented too.The safety factor of blocks is calculated by strength reduction technique and limit equilibrium theory,and the characters of errors are pointed out when it is calculated using limit equilibrium theory.