The invention discloses a device and a method for simultaneous evaporating and diluting of ammonia water by using hot blast of a boiler, which solves the problems of high cost of ammonia gas preparation system and blockage of spray nozzle in the prior art. The device comprises an ammonia water storage tank, a compressed air storage tank, an ammonia water evaporator and a boiler hot blast air duct, wherein the ammonia water storage tank is used for storing ammonia water; the compressed air storage tank is used for providing compressed air; a double-fluid spray gun is arranged in the ammonia evaporator; the ammonia water storage tank and the compressed air storage tank are respectively and independently connected with the double-fluid spray gun, and the ammonia water is atomized by the double-fluid spray gun; the boiler hot blast air duct is connected with the ammonia water evaporator by a pipeline, a hot blast fan is arranged on the pipeline and is used for sending the boiler hot blast into the ammonia water evaporator, and a hot blast inlet of the ammonia water evaporator is lower than the location of the double-fluid spray gun; the atomized ammonia water is evaporated and vaporized under the action of hot blast, and the vaporized ammonia water is diluted by hot air.