1. The invention relates to the technical field of coal-fired flue gas treatment, in particular to a coal-fired flue gas treatment composition and a treatment method, device and application thereof. The composition comprises a carbonic acid solution, a copper nitride solution and a sodium chloride solution which are respectively used; Carbonic acid solution consists of sodium bicarbonate, sodium carbonate and solvent; The nitrogen copper solution consists of mother liquor and seed liquor, wherein the mother liquor is made of heated copper and urea solution, and the seed liquor is urea solution. 3. The coal-fired flue gas treatment composition designed by the invention can reduce the NO content in the coal-fired flue gas to within 10mg/m, thus improving the degradation level of nitrogen oxides to a brand-new level.