The invention discloses a reducing agent spraying and metering device, which comprises a reducing agent storing box, a supply pump, a metering valve, a mixing cavity, an atomizing nozzle, a controller, a switching valve and a low-pressure air pump, wherein a reducing agent of the reducing agent storing box is mixed in the mixing cavity through the supply pump and the metering valve, is then atomized through the atomizing nozzle, is sprayed into an exhaust pipe, is mixed with waste gas, and is reduced and purified; the low-pressure air pump is used for supplying pressure air to the mixing cavity during working of an engine, and sweeping the reducing agent in the supply pump and a pipeline behind the supply pump back into the reducing agent storing box when the engine stops working; and the switching valve is used for controlling and switching the reducing agent storing box to supply the reducing agent and pressure gas of the low-pressure air pump to sweep the supply pump and the pipeline behind the supply pump. An independent air source is adopted, so that the pressure is stable and does not fluctuate along with the change of the working condition of the engine, automobile compressed air is not consumed, and the potential safety hazard of compressed air leakage is eliminated. A cleaning method of the spraying and metering device is simple, and a complete effect is achieved.