To meet the high maneuverability and rapid response demands of modern helicopters, it is crucial to examine the dynamic characteristics of key subsystems as an integrated system. This study develops a coupled dynamic model of the rotor–engine–transmission system, taking into account the influence of the rotor and engine on the transmission system. The effects of forward flight speed and rotor rotational speed on the vibration and load-sharing characteristics of the transmission system are analyzed. Results show that with increasing forward flight speed, the torsional vibration amplitude value of all gear pairs first decreases and then increases, with vibration magnitude of all gear pairs exhibiting a similar trend with vibration amplitude value. Correspondingly, load-sharing performance initially improves and then deteriorates. In contrast, increasing rotor speed continuously amplifies both the torsional vibration amplitude value and vibration magnitude, while progressively worsening load-sharing performance. However, variations in forward flight speed and rotor speed do not alter the distribution of vibration frequency components or the dynamic behavior patterns of the gear pairs. This research offers a new approach and methodological framework for investigating main reducer dynamics at the full-aircraft level, contributing valuable insights to helicopter system design and performance analysis.