Based on the underground powerhouse of Dagangshan Hydropower Station, the characteristic angles according to the geometric relationship between the principal stress direction and the cavern space position are defined, in order to describe the rotation of the principal axes of stresses. The research shows that different degrees of stress release and principal axis rotation are induced in the arch and the side wall during the excavation. The principal stresses in surrounding rock unload intensely in the current excavation, while the direction of principal stresses rotates violently in the current and subsequent excavation stages. Furthermore, the principal stress direction adjustment has certain similarities in the arch and the side wall. The final direction of the minimum principal stress is approximately perpendicular to the cavern free surface, and the directions of the maximum principal stress and intermediate principal stress are adjusted almost parallel to the excavation surface plane. On this basis, considering the effect of principal stress rotation, the stress disturbance index (SDI) is defined as a mechanical characterization quantity, and using it to describe the disturbance degree of stress field in the surrounding rock. SDI is applied to study the time evolution, along with the space distribution characteristics of stress field disturbance in the underground powerhouse of Dagangshan Hydropower Station. The strong stress disturbance zone obtained by numerical simulation is compared with the excavation damaged zone by field test, and the related understanding and conclusions are of certain theoretical and engineering significances.