In response to the application requirements of low-speed heavy-load and complex working conditions of new energy heavy vehicles, this paper takes new energy heavy vehicles as the research object and conducts the optimization design of the main reducer gear of the electric drive axle. Considering the characteristics of low-speed heavy-load and continuous operation of the vehicle, a parallel-axis three-stage helical cylindrical gear transmission structure is adopted to complete the layout and key parameter optimization design of the reduction and torque-increasing gear pair. Through the combination of finite element strength simulation and bench test, the maximum power condition and reverse driving condition are selected as the extreme conditions for strength analysis, and 300000 fatigue durability tests are completed according to industry standards. The results show that the strength and durability performance of this main reduction gear meet the usage requirements, and the design scheme is reasonable and reliable, which can provide a reference for the independent research and engineering application of the electric drive axle of new energy heavy vehicles.