The electric helicopter offers substantial advantages, including energy efficiency, reduced emissions, and noise reduction, thus paving the way for innovation and technological advancement in the aviation industry. Ensuring the electric propulsion system (EPS) can effectively withstand dynamic loads is crucial for the safety and operational performance of electric helicopters. This study examines key physical characteristics, such as shaft flexibility, time-varying meshing properties of the gear transmission system, harmonic and saturation effects of the permanent magnet synchronous motor (PMSM), and the impact of dynamic loads on the motor control system. Additionally, the natural characteristics, including natural frequencies, modal shapes, and forced torsional vibrations, are analyzed with consideration of electromagnetic stiffness. Establishing a model of the EPS electromechanical coupling system, the relationship between torque ripple and harmonic current is explored, and a suppression method based on harmonic current commands is proposed. A high-order current harmonic controller is designed to ensure that the actual current is dynamically corrected in real time to follow the harmonic current command. Simulation results demonstrate that this method effectively reduces the harmonic torque amplitude of the PMSM, significantly mitigating torsional vibration in the transmission system and enhancing the stability of power transmission.