The gears in aero-engines perform energy and motion transfer between the High-Pressure Rotor (HPR) and accessories. Firstly, an occurred fault with radial cracks in the driven gear disk was diagnosed as High Cycle Fatigue (HCF) failure through necessary examinations. Analysis of fault tree and test signals indicated that the fracture was relevant to the swing vibration of the driven gear excited by the unbalance excitation on HPR. Secondly, a corresponding mecha-nism model was established, in which the gear meshing effect was considered, with lateral and swing vibration mode. The Governing Equations (GEs) and the Finite Element Model (FEM) were estab-lished and verified. Then, the modal shapes, harmonic response, and transient response were ana-lyzed, indicating that the swing vibration of the driven gear could be significantly excited by the unbalance on HPR, which induced the cracks in the driven gear disk to extend radially. Further-more, influences of factors on the unbalance response were obtained, in which the unbalance response appeared local minimum points and maximum points. Meanwhile, 1# and 2# bearing stiff-ness had rather significant influences on the response. Thus it is efficient to modify them to achieve vibration control.