This study investigates torsional vibration suppression in helicopter electric propulsion systems (EPS) through harmonic current control. Helicopter electrification introduces critical transmission reliability challenges from electromagnetic torque ripple-induced vibrations. Conventional helicopter solutions prove inadequate for electric systems due to fundamental differences in electromechanical coupling dynamics. We develop an integrated electromechanical model combining a permanent magnet synchronous motor (PMSM) with a two-stage gear transmission (total ratio 17.44:1). Dynamic analysis reveals spectral coupling in the first-stage gear mesh forces, containing second-stage (573.4 Hz) and PMSM current harmonic (202 Hz) components, while the second stage demonstrates spectral isolation attributable to reduced inertia and motor decoupling. To mitigate torque ripple from 5th, 7th, and 11th current harmonics, we embed a harmonic suppression module within the PMSM vector control architecture. This strategy injects compensating voltages to neutralize harmonic distortions, achieving near-sinusoidal stator currents. Simulations confirm: (1) 15 Nm reduction in high-speed shaft torque amplitude, (2) elimination of 6th-order harmonic components. The approach effectively attenuates torsional vibrations under shock loads, enhancing system stability. Key contributions establish: (1) Motor torque pulsations critically excite gear mesh dynamics through spectral coupling phenomena; (2) Active harmonic suppression significantly reduces electromechanical vibrations. This work delivers a flight-worthy control framework for electric helicopter EPS, resolving high-torque electromechanical coupling challenges in aerospace propulsion systems.