The invention discloses a method for desulfurizing high-concentration flue gas. The high-concentration flue gas is desulfurized through a sodium sulfite method, and meanwhile, the sodium sulfite is recovered as a by-product, wherein the flue gas needs to be deoxidized before being desulfurized through the sodium sulfite method. In the method for desulfurizing high-concentration flue gas disclosedby the invention, by deoxidizing the flue gas through an appropriate pretreatment process, the purity of the by-product sodium sulfite produced by the subsequent flue gas desulfurization process of the sodium sulfite method can be obviously improved, and the value of the by-product is improved, thereby obviously improving the comprehensive economic benefits. The pretreatment process of the flue gas is performed by using the sulfite and/or the hydrosulfite produced by the reaction of SO2 contained in the flue gas and alkali to react with oxygen without using any other deoxidizer. The method disclosed by the invention has simple production process and is easy to operate.