The invention relates to a gas-electric hybrid propulsion system for an airplane. The gas-electric hybrid propulsion system for the airplane comprises an engine, a throttle rod sensor, an atmospheric parameter sensor, an energy management module, a conventional fuel module and a lithium battery pack. The throttle rod sensor is used for sensing an airplane power demand signal and connected to the energy management module to transmit the airplane power demand signal to the energy management module. The atmospheric parameter sensor is used for sensing atmospheric parameters and connected to the energy management module to transmit the atmospheric parameters to the energy management module. The conventional fuel module and the lithium battery pack are connected to the energy management module and the engine. The engine is driven by the conventional fuel module or the lithium battery pack, which is determined by the energy management module according to the airplane power demand signal from the throttle rod sensor and the atmospheric parameters from the atmospheric parameter sensor. The gas-electric hybrid propulsion system for the airplane has the following advantage of being capable of meeting relevant requirements of being economic and environmentally friendly for high-performance engines.