The invention provides a scanning defrost type air-source gas heat pump, which comprises a gas engine, a Freon compressor, a four-way valve, an indoor heat exchanger, a throttling valve and an outdoor heat exchanger, and is characterized by also comprising a smoke three-way valve and a smoke scanning defrost device, wherein the smoke scanning defrost device is a small-area device which can circularly move along a windward side of the outdoor heat exchanger and jet smoke to a frosted heat transfer surface of the outdoor heat exchanger; the input end of the smoke three-way valve is connected with a smoke exhaust opening of the gas engine; and one output end of the smoke three-way valve is connected with the gas inlet end of the smoke scanning defrost device through a smoke pipe. High-temperature smoke exhausted from the gas engine performs scanning quick defrosting on the outdoor heat exchanger partially and circularly, so that the air-source gas heat pump can defrost and work normally without stopping at a low temperature or an ultralow temperature in the absence of defrosting energy loss. Therefore, the scanning defrost type air-source gas heat pump is an upgraded product of the conventional air-source gas heat pump.