Abstract: It is easy for the columnar jointed basalt to relax after excavation, which has important influences on the deformation of arch dam and the stability of the surrounding rock of underground caverns. Therefore, the relaxation of the columnar jointed basalt is one of the main rock mechanics problems in Baihetan Hydropower Station. It mainly includes three types: unloading relaxation, fracturing relaxation and collapse relaxation. The relaxation of columnar jointed basalt is monitored by using the borehole acoustic wave tests as the main monitoring method and the borehole TV panoramic imaging tests as the auxiliary monitoring method. It is found that the excavation-induced relaxation has significant time effects and space effects. The time effects of relaxation can be represented by the logarithmic function, and the relaxation tends to be stable after about 35 days of excavation. The macroscopic relaxation failure is closely related to timeliness and limitation of support. The relaxation depth, especially for side walls, increases gradually with the increase of the cavern span, excavation height and cross-sectional area. The relaxation of the columnar jointed basalt is the combined effects of lithology, structural plane, rock mass structure and in-situ stress. A series of control technologies are put forward, and good control effects are achieved.