<p>Exploring the excitation interference and coupling mechanisms in the multi-stage gear system is crucial for unraveling the dynamic characteristics of the aero-engine accessory gearbox (AGB). To precisely describe the dynamic behavior of the AGB system, this study develops a multi-degree-of-freedom (MDOF) nonlinear gear dynamic model to investigate the interaction of multiple internal excitations. The model incorporates time-varying meshing stiffness, comprehensive meshing error, and tooth-surface friction excitation together with the actual load-transfer characteristics of the AGB. A representative gear-pair experiment and numerical analyses are conducted to evaluate the dynamic response, natural characteristics, and operating-condition-dependent behavior of the system. The results show that the AGB system exhibits pronounced time-varying dynamics under multiple excitations. Time-varying meshing stiffness governs the basic periodic variation of the response, meshing error intensifies transmission fluctuation, and friction excitation mainly affects the directionality and amplitude of vibration. In addition, the modal properties and meshing-frequency coincidence jointly determine the local vibration sensitivity of different gears under different operating conditions. These results provide system-level insight into multi-excitation coupling in complex aero-engine AGB transmissions and support vibration control and condition monitoring.</p>