A hybrid power system, comprising a first power unit, a secondary shaft, a first transmission device, an auxiliary power unit, and a second transmission device. The first power unit has a primary shaft. The secondary shaft is driven by the primary shaft in a rotational movement. The first transmission device is placed between the primary shaft and the secondary shaft, having a transmission belt, transmitting torque from the primary shaft to the secondary shaft. The auxiliary power unit has an electric motor with a driving shaft, which is parallel to the primary shaft. The second transmission device is placed between the primary shaft and the driving shaft, transmitting torque from the electric motor to the primary shaft.