Abstract: With increasingly severe problems of energy consumption and environmental pollution, under the guidance of fuel consumption and emission regulations, how to further reduce fuel consumption has become the top priority of the development of the automobile industry. With a consideration of both power and economy, Plug-in Hybrid Electric Vehicles (PHEV) have lower fuel consumption than traditional fuel vehicles and have multiple power drive modes in their design. Compared with pure electric vehicles, they do not have the problem of endurance mileage anxiety, and become the focus of the current automobile industry. However, fuel economy of PHEV requires a careful consideration of various factors. How to improve fuel economy has been the focus and difficulty of the automotive industry. The PHEV based on the P2.5 hybrid system configuration integrates drive motors into a particular input shaft of transmission, with high drive and braking efficiency. It can drive directly through the clutch and transmission and work together with the engine. Compared with other hybrid models, the P2.5 configuration PHEV has higher integration, less space, a smooth connection between oil and electricity, higher drive and braking efficiency, and superior fuel economy. Therefore, this paper takes the P2.5 configuration PHEV as the research object and proposes a rule-based energy management strategy to improve fuel economy of the P2.5 configuration PHEV. First of all, the power system of PHEV with P2.5 configuration is analyzed, and, combined with the advantages of this configuration, it is divided into five working modes: pure electric driving mode, pure engine driving mode, light load charging mode, hybrid driving mode, and regenerative braking mode. Secondly, based on the idea of the logical threshold value of the state of charge (SOC) of the power battery, a multi-phase energy management control strategy of CD-CS is proposed. According to the change trajectory of the SOC value of the battery, the active phase of PHEV is divided into two phases,namely charge-depleting (CD) and charge-sustaining (CS), and corresponding rules are formulated according to the SOC value of the battery to switch the two modes. On the premise of meeting the power, the vehicle is operated to use up the battery energy as much as possible to reduce fuel consumption within the driving range. Finally, according to the idea of the forward simulation model, the driver model, engine model, drive motor model, power battery model, vehicle dynamics model and other vital components of PHEV are simulated under different working conditions in Matlab/Simulink software based on experimental data. The results show that the vehicle model established in this paper and the proposed rule-based energy management strategy are accurate and effective. Compared with the pure engine mode,fuel savings of 100 km under WLTC, NEDC, UDDS and HWFET conditions increases by 34.6%, 38.8%, 63.3%, and 30.9%respectively. The simulation results show that fuel savings under all conditions increase by more than 30%, with a maximum increase of 63.3%. This fully demonstrates that the rule-based energy management strategy can effectively improve fuel economy of the engine.