The invention discloses a hybrid power system for a fixed-wing unmanned aerial vehicle and a propelling method. The system comprises a first propulsion motor, a first propeller, a first ISG motor, a first ISG motor clutch, a second propeller, a power controller, a battery pack, a transmission shaft, a third propeller, a second ISG motor clutch, a second ISG motor, a fourth propeller, a second propulsion motor, a first bevel pinion set, a transmission shaft bevel gear set, a second bevel pinion set, a main propeller, a main propeller clutch, a fuel engine and a transmission shaft clutch. The hybrid power system combines the advantages of fuel propulsion and pure electric propulsion, has multiple hybrid propulsion modes, and can effectively improve the whole voyage of the fixed-wing unmanned aerial vehicle.