In this paper, the mechanism of the asynchronous vibration response phenomenon caused by the looseness fault in the aero-engine whole vibration system is investigated by numerical integration methods. A single degree-of-freedom (DOF) lumped mass model and a rotor-casing whole vibration model of a real engine are established, and two looseness fault models are introduced. The response of these two systems is obtained by numerical integration methods, and the asynchronous response characteristics are analyzed. By comparing the response of a single DOF lumped mass model with the response of multiple DOF model, the reason leading to the asynchronous response characteristics is the relationship between the changing period of stiffness and the changing period of the rotational speed. When the changing period of stiffness is equivalent to the changing period of the rotational speed, frequency multiplication will appear and the natural frequency will be excited at specific speeds. When the changing period of stiffness is equivalent to n (n = 2, 3,…) times the changing period of the rotating speed, 1/n (n = 2, 3,…) frequency division and frequency multiplication will appear and the natural frequency will be excited at specific speeds.