<p>With the continuous development of hybrid vehicles, the emergence of a multi-speed hybrid transmission that is both efficient and intelligent is expected to become an unavoidable direction for future development. The compact and complex gear transmission system in hybrid powertrain transmissions is susceptible to issues related to gear vibration, system vibration, and noise. Therefore, it is highly important to conduct research on optimizing modifications for the gear. The research object of this paper would be the gear transmission system of a four-gear hybrid transmission of parallel shaft helical gear type. The design of the gear transmission system model was proposed and the accuracy of the model was checked through experiments. This study examined how altering the micro-shape of gears affects their performance during meshing, considering various torque loads and modification techniques; the composite modification scheme of gear was determined in this article, the variation of gear contact spot, tooth surface contact stress, and the analysis and comparison of peak-to-peak transmission error were conducted in this paper, both before and after the modification. The revised results show that the tooth surface contact stress reduces by 28.5%, the peak-to-peak transmission error decreases by 43.8% after modification, and the state of gear pairing impact and gear deviation loading is bettered after the alteration. This paper provides a theoretical reference for the gear modification of hybrid powertrain transmission.</p>