The invention relates to a method and a device for providing a reducing agent for a full operating condition of an automotive selective catalytic reduction system. According to the method, ammonium carbonate is heated by adopting an electric heating fan to generate ammonia gas. The device mainly consists of a closed container, the electric heating fan, capacity monitoring sensors and an electronic control valve, wherein the closed container is used for holding the ammonium carbonate. Ammonium salt is added to the closed container by a charging hole, and the closed container is divided into two independent closed spaces by a partition plate. The partition plate not only has a function of blocking the two closed spaces but also has a function of a heat transfer plate. A thermal insulating layer is wrapped on the outer layer of a main body of the device. The electrical heating fan is arranged in one of the two closed spaces, and is fixedly arranged at the bottom part of the device by a support. The heat transfer plate is semicircular so as to increase heating area, and therefore, the flow of internal circulating air is enhanced, and heat transfer is enhanced. The ammonium salt enters an ammonia gas jet apparatus of an after-treatment system of an engine by the electronic control valve after being heated to be decomposed. The method and the device for providing the reducing agent for the full operating condition of the automotive selective catalytic reduction system have the characteristics that the ammonia gas can be rapidly and efficiently generated, and the gas production rate is changed along with the operating condition change of the engine.