The investigation of stiffness degradation, pretension force evolution, and system instability mechanisms in bolt - bearing - high pressure rotor systems is crucial for enhancing the engineering applicability of dynamics studies in high - speed rotating machinery. Firstly, the evolution of connection stiffness and pretension force in the bolt - bearing - high pressure rotor system is investigated. The influence of bolt pretension force (radial and axial) on the stiffness characteristics of rolling bearings and rotor systems is analyzed. The results demonstrate that the stiffness of rolling bearings exhibits a significant nonlinear evolution pattern of first increasing and then decreasing with the increase of radial bolt pretension force. This is due to the evolution of the structural contact state caused by interference fit assembly. In contrast, the axial pretension force exerts a comparatively minor influence on the stiffness of the bearing system. Then, the mechanism of its effect on the dynamic performance of the rotor system, such as critical speed and vibration mode, is analyzed. The research results show a strong positive correlation between the critical speed of the rotor system and the bolt pretension force. It quantitatively elucidates the influence of pretension force on the stiffness of key components and system dynamics, providing the theoretical basis for the structural optimization design, the formulation of precision assembly process parameters, and the identification of vibration faults in systems.