Abstract: In recent years, some large and extra large hydropower projects have been built or are under construction in China. Underground caverns of these hydropower projects usually have problems of complex geological conditions and high geostress. High geostress often leads to failure of different degrees and relaxation of extraordinary depth in surrounding rock during the construction of caverns. In this paper, the development of relaxation in surrounding rock is studied through analyzing geophysical prospecting data while excavating of main powerhouse at Jinping I hydropower station. Optimal supporting time is studied through analyzing displacement and plastic zone of the surrounding rock through numerical analysis. It is shown that the plant axis should be parallel with the maximum principal stress, which can effectively reduce relaxation depth caused by the time-dependent deformation.80% of the relaxation depth in surrounding rock is formed after the excavation interval of 2 to 3 layers. Compared with the backward support, the displacement and plastic zone are obviously smaller when immediate support is applied. So, immediate support is beneficial to the stability of the surrounding rock. These research results can be used for reference when selection of plant axis, support design and supporting time determination of large underground caverns under high ground stress.