The invention discloses a coating material used for reducing the SO2/SO3 conversion rate of an SCR denitration catalyst, a method and a unit box.The coating material comprises, by weight, 0.5-10% of rare earth metal oxide, 1-20% of alkaline earth metal oxide and the balance substrate materials.The coating material has the advantages that the coating material can effectively inhibit deposition of rust and the iron element on the surface of the denitration catalyst in the production and regeneration process of the SCR denitration catalyst, reduces the SO2/SO3 conversion rate in the denitration reaction process, and improves stability and denitration efficiency of the denitration catalyst; the inner wall of the unit box is coated with the material, then denitration catalyst single plates are loaded, corrosion on the inner wall of the unit box and equipment used in the denitration process can be effectively prevented, and the aims are achieved.The method is easy to operate, low in cost, suitable for plate-type denitration catalysts of various types and with various components and particularly suitable for plate-type denitration catalysts needing to be regenerated.