Understanding the mechanism of rotor looseness in aircraft engines and the characterization of the corresponding rotating mechanical parameters play an important role in quickly diagnosing technical problems about rotor looseness and improving the stability design of the rotor. Taking a certain rotor with curvic couplings as the research object, this paper analyzed the mechanism of the influence of preload and loosening changes on the stability of the rotor structure by applying the validation methods of numerical simulation and modal test. The results showed that the changing trends in simulation calculation and modal test were the same. When the preload of the rotor decreased, the stiffness of curvic couplings sharply decreased, and the critical bending speed of the rotor decreased significantly. Therefore, when the rotor with curvic couplings became loose, its operating characteristics indicated a typical resonance instability phenomenon. In rotor dynamic property design, the stiffness of the rotor with curvic couplings should be designed according to the minimum preload during the operating process. Before installation tests, modal tests should be conducted on rotors with different preload states to obtain the minimum allowable preload value during operation and verify whether the design meets the application requirement.