The dynamic characteristics of the helicopter main reducer bevel gear have a direct influence on the operating performance of the helicopter transmission system. Taking into account factors such as transmission error and backlash, a nonlinear damage dynamic model of a bevel gear system with bending-torsion-axial coupling was established by using the lumped-parameter method. In order to obtain the key parameter in the damage dynamics model, a slicing method for calculating the time-varying meshing stiffness of spur bevel gear pair was proposed, which broke through the limitations of traditional potential energy method that can only be applied to cylindrical gear pair and can achieve rapid evaluation of time-varying meshing stiffness under different states. By comparing the model simulation and experimental results, it is shown that the established model can effectively simulate the dynamic response characteristics of the bevel gear system under normal and fault conditions, reveal the system fault response mechanism and fault mechanism, and provide theoretical basis and data support for the development of a helicopter transmission system health and usage monitoring system.