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The research took a dual-axis-parallel plug-in hybrid electric vehicle (PHEV) as its research object: the engine and electric motor were connected with an automated mechanical transmission (AMT) through two different axles. As the torques of the engine and the electric motor converge through a variety of gear combinations selected within the AMT, the gearshift schedule has a high degree of coupling with the energy management strategy and therefore they jointly determine various aspects of the vehicle performance. Under the coupling constraints of torque and speed of the engine and electric motor, an instantaneous optimal energy management strategy was proposed by considering the influences of torque distribution and gear-matching on the performance of the whole vehicle. By adjusting the torques of the engine and the electric motor, as well as the gear of the AMT independently, the strategy allowed both the engine and the electric motor to operate at a higher efficiency. In this way, the engine, the electric motor, and the AMT can be comprehensively controlled and optimised. The feasibility and effectiveness of the proposed integrated optimisation strategy was proved by simulation on the AVL Cruise and MATLAB™/Simulink platform.