The main reducer is the core component of the helicopter transmission system, and its dynamic performance directly affects the safety, reliability and comfort of the helicopter. However, due to the complex structure and working conditions of helicopter main reducer, there is still a lack of research on the influence of working conditions on the vibration characteristics of the helicopter main reducer. Aiming at the above problems, this paper investigates the effects of working conditions such as system speed, rotor torque and rotor lift on the vibration characteristics of helicopter main reducer. The main excitation that causes the vibration of the helicopter main reducer is revealed. It is found that the vibration of helicopter main reducer increases with the increase of rotor torque. The system speed has a significant effect on the vibration characteristics of the helicopter main reducer. The rotor lift has influence on the vibration characteristics of the main reducer, but the influence is not great. The meshing frequency of each gear pair of helicopter main reducer is mutually coupled and transmitted. The meshing frequency of each stage is usually the most dominant excitation frequency of the corresponding stage. The high-speed stage vibration of the helicopter main reducer is the largest, the inter-speed stage vibration is small, and the low-speed stage vibration is the least. When improving and optimizing design, it is important to focus on the high-speed and planetary stages of the helicopter main reducer. This study can provide the basis and guidance for the vibration reduction design and dynamic performance optimization of helicopter main reducer.