Abstract: One of the most effective methods for the removal of NO_z from industrial flue gas is the technology known as low-temperature selective catalytic reduction (SCR). The main problem limiting the industrial application of catalysts is the need to improve their performances at low temperatures, and the fact that the anti-toxic mechanism of low-temperature denitration catalysts has yet to be explicitly identified. In this study, a TiO₂−ZrO₂−CeO₂ (molar ratio 4∶1∶1.25) carrier was prepared by the sol–gel method, and then loaded the active components MnO_x and MnO_x−FeO_y using the citric-acid-solution impregnation method to synthesize a new type of Fe-modified Mn-based multi-oxidation-state composite catalyst. The performance of this Mn-based composite oxide catalyst was investigated with respect to its NH₃-selective catalytic reduction of NO and sulfur resistance. The catalyst exhibits good low-temperature SCR redox ability and anti-poisoning ability in an SO₂-containing atmosphere, whereby the introduction of Fe promotes the interaction between Mn and the TiO₂−ZrO₂−CeO₂ (4∶1∶1.25) carrier, and increases the number of Lewis acid sites on the catalyst surface. According to the XPS analysis, the contents of Mn⁴⁺, Ce⁴⁺, and adsorbed oxygen are obviously increased, which is very advantageous for improving the performance of the catalyst. According to the thermogravimetric analysis, the introduction of Fe reduces the production of ammonium sulfate and ceric sulfate in the atmosphere containing SO₂ and H₂O, and inhibites the sulfation of manganese. The Fe element thereby increases the anti-toxic ability of the Mn-based multi-oxidation-state composite catalyst. By maintaining the MnO_x (12.5%)−FeO_y(0.8)/TiO₂−ZrO₂−CeO₂ (4∶1∶1.25) catalyst at 180 ℃, while continuously feeding 10% H₂O in volume fraction and 125×10⁻⁶ SO₂ for 240 min, the NO_z conversion rate can be stably maintained at 75.6%. Based on the results of this work, a new type of Mn-based composite oxide catalyst has been developed that provides a foundation for further exploring the SCR reaction of the catalyst and its anti-toxic mechanism to promote the industrial application of the SCR process.