Abstract: In order to explore the influence of intermediate principal stress on the mechanical properties of prefabrica-ted holes in rock,the authors carried out the constant b and σ3 tests of prefabricated hole in sandstone under true triax-ial stress conditions based on the self-developed “ multi-functional true triaxial geophysical ( TTG) apparatus”. The experimental results show that the principal stress axis direction and the intermediate principal stress all have great in-fluence on the deformation and strength characteristics of the rock. The crack characteristic stress increases first and then decrease with the increase of the principal stress coefficient b. In addition,the crack develops mainly in the direc-tion of the vertical borehole axis and begins to destroy in the direction of the smaller principal stress. The strength of the specimen increases first and then decreases with the increase of the principal stress coefficient in several test con-ditions. Among them,the highest strength and the best stability occur when the maximum principal stress is parallel to the borehole axis. The rock strength is the lowest when the intermediate principal stress is parallel to the borehole axis at a smaller value of b. In contrast,the strength is the lowest when the minimum principal stress is parallel to the borehole axis at a larger value of b. Furthermore,the authors analyzed the effects of intermediate principal stress on the plastic zone around the hole based on the generalized plane strain theory.