Due to the complexity of seismic coupling between vehicles and highway bridges， current research on the mechanism of vehicle-bridge interaction under seismic action is relatively scarce， leading to diverse specifications about whether live load should be taken into account in national seismic bridge design codes. Existing studies have shown that vehicles possibly would produce beneficial effect on bridge seismic responses in one circumstance but adverse effects in another. The simulation of contact behaviors between vehicle and bridge is simplified in this study. On this basis， the mechanism of vehicle-bridge interaction is analyzed thoroughly by employing the energy method. Analysis results demonstrate that the impact of vehicle-bridge interaction on horizontal energy input to the bridge is negligible.However the interaction can reduce the total energy input to the bridge in the vertical direction， therefore reducing the vertical seismic responses of the bridge， while input energy to the vehicle’s vertical direction comes mainly from the bridge’s vibration. On the other hand， the smaller the vehicle mass， the greater the horizontal input energy to the vehicle caused by bridge vibration.