A series of indium-modified manganese oxide catalysts with different ratios were prepared by the homogeneous precipitation method. The appropriate amount of indium modification has many effects on the MnOxcatalysts, such as increasing the specific surface area, increasing the concentration of surfactant Mn4+and surface oxygen, enhancing the surface acid site strength, and improving the low-temperature activity of the MnOxcatalysts. the Mn-InO-6 catalyst (Mn: In=6:1) showed the best activity, maintaining approximately 100% NOxat conversion rates of 75-200 °C. Indium doping significantly improved the sulfur resistance of the MnOxcatalyst with high reversibility and good stability. In situ DRIFTs indicate that the NH3-SCR reaction over Mn-InO-6 catalysts follows the Eley-Ridel and Langmuir-Hinshelwood mechanisms.