As a critical way to transfer power from the engine to the rotor and accessories, the performance of the helicopter transmission system directly impacts the helicopter’s stability and safety. In particular, the aerodynamic excitation of the main rotor significantly affects the torsional vibration of the transmission system. Therefore, it is important to characterize the helicopter transmission system's torsional vibration under the main rotor's excitation. This paper, a complete helicopter transmission system model is established to give an insightful understanding of the system dynamics. An in-depth analysis of the system's torsional vibration characteristics under main rotor excitation is conducted. The analysis results show that compared with the state without aerodynamic load excitation, when the main rotor aerodynamic load is introduced as an excitation source into the transmission system, the excitation can cause the first-order torsional vibration of the system. The amplitude of the torsional vibration response at each measurement point increases, which easily leads to the instability of the transmission system.