Abstract: For the universal phenomenon of aero-engine connectors with looseness fault, a single degree of freedom lumped mass model was established and a looseness fault model was introduced. The response of the system was obtained by numerical integration methods and the asynchronous response characteristics are analyzed. The experiments were conducted on the connectors with looseness clearances. It is found that the acceleration response of a mass block after noise reduction has up-down asymmetrical impact characteristics in a waveform, also the pseudo-critical subharmonic resonance and the pseudo-critical ultra-harmonic resonance appear in a frequency spectrum. These characteristics are in an agreement with the results of the numerical simulation, which can be identified as the characteristics of looseness faults. The reason leading to the looseness characteristics is that the period of stiffness changes in the period of rotating speed. When the changing period of stiffness is equivalent to the vibration period, the frequency multiplication will appear and the critical frequency of the system will be excited at specific speeds. When the changing period of stiffness is equivalent to n times of the vibration period, 1/n frequency division and frequency multiplication will appear and the critical frequency of the system will be excited at specific speeds.