This paper examines the mechanism of asynchronous response characteristics caused by looseness faults in aero-engine support systems. First, a single-degree-of-freedom rotor model without mass was established, and a looseness fault model was introduced. The response of the system was obtained by the numerical integration method, and the asynchronous response characteristics were analyzed. Second, an entire engine rotor-bearing-casing model was established, and a looseness fault model was introduced. The coupled system response was solved with the numerical integration method, and its asynchronous response characteristics were analyzed. The results showed the reasons that support looseness faults in aero-engines caused frequency division and frequency multiplication: When the changing period of stiffness was equal to that of the rotation speed, frequency multiplication appeared, and the corresponding frequencies of critical speeds were excited at certain speeds. Moreover, when the changing period of stiffness was integer times that of the rotation speed, 1/n times frequency division and frequency multiplication appeared, and the corresponding frequencies of critical speeds were excited at certain speeds.