The invention discloses an impeller, a water flow switch, a heat exchanger and an air energy heat pump water heater. The impeller capable of accurately detecting the flow of a pipeline comprises a huband blades, and the side edges, connected with the hub, of the blades intersect with plain lines of the outer circumferential surface of the hub. The impeller can accurately detect the flow in the pipeline, so that the action of the corresponding water flow switch is more accurate, and the safety of the whole heat exchanger and the whole air energy heat pump water heater is protected.