To improve the energy efficiency of electric commercial vehicles and fully utilize their energy-saving potential, this study utilizes multi-gear automated mechanical transmission (AMT) electric heavy trucks as the research object and proposes a shifting strategy based on the optimal system efficiency. First, based on the electric drive system configuration, the loss mechanism and dynamic efficiency characteristics of each component are analyzed, and the shifting strategy based on the optimal system efficiency is formulated through the electric drive system efficiency surface. Second, a bond graph model of the electric drive system is established, and the simulation analysis of the shift strategy is carried out by using the reconstructed suburban working conditions. Finally, the effectiveness and superiority of the proposed strategy are verified through the real vehicle experiments. The research results indicate that the proposed strategy achieves real-time control of the optimal output efficiency of the electric drive system, and reduces the overall vehicle energy consumption by 3.86% compared with the traditional economic gearshift strategy.