The invention discloses an intermediate temperature efficient modified SNCR denitration device of a layer-burning boiler and a method, and belongs to the technical field of coal-fired boiler pollutant control. Amino and imido free radicals are generated by urea under the synergistic effect of H2O2 and organic acid (formic acid or acetic acid or propionic acid), and due to the fact that amino free radicals .NH2 or imido free radicals .NH has higher reaction activity and selectivity on nitric oxide, a temperature window of an SNCR reaction can be greatly reduced; meanwhile, a modified reducing agent atomizing nozzle is located in a furnace outlet turning room of the layer-burning boiler, sufficient mixing of a modified reducing agent and nitric oxide can be assisted by the area, and thus the utilization rate of urea is increased. The device and the method are economical and environmentally friendly, implementation is easy, and the denitration efficiency is high; besides the layer-burning boiler, the device and the method are also applicable to coal-fired power plants and other coal-fried industrial processes, and have wide application prospects and high market popularization value.