The invention discloses an energy-saving and environment-friendly air-fuel hybrid engine and belongs to the technical field of engines. The energy-saving and environment-friendly air-fuel hybrid engine mainly comprises a first fuel engine air cylinder (1), a second fuel engine air cylinder (2), a compressed air cylinder (15), a compressed air expansion cylinder (5) and a heat exchanger (9). The heat exchanger (9) collects hot gas exhausted from two fuel engine bodies and normal-temperature gas exhausted by the compressed air cylinder (15), and the hot gas and the constant-temperature gas are reheated and exhausted into the compressed air expansion cylinder (5) to act. Thus, utilization of waste heat of the waste gas of the fuel engine bodies is achieved. According to the energy-saving and environment-friendly air-fuel hybrid engine, a gas storage tank is arranged for standby application, and during braking, compressed air enters the gas storage tank; during starting and at idle, the compressed air of the gas storage tank is used for driving the compressed air expansion cylinder to complete operation; during starting and at a low speed, the pneumatic engine is used, so that energy is saved, exhaust emission is avoided, the service life of the pneumatic engine is the same with that of a traditional engine, a battery does not need to be replaced regularly, cost is low, refitting is easy, and the cost advantage is achieved.