The MnFe/TiO2 catalysts were poisoned by metal sulfates and/or ammonium sulfate to understand the SO2 poisoning effect under low-temperature selective catalytic reduction (SCR) of NO with NH3. The results showed that the formation of metal sulfates had a more serious deactivation effect than that of ammonium salts on the MnFe/TiO2 catalysts. After thermal regeneration, the metal sulfates on the catalyst could not be removed so that the NOx conversion was only recovered from 17% to 35%. On the other hand, water washing was capable to remove both ammonium salts and metal sulfates, and the NOx conversion could be recovered to 88% (compared to 99% for the fresh catalyst). The analytical results of the synchrotron-based XRD, BET, NH3-TPD, and XPS revealed that lower crystallinity, lower specific surface area, lower ratio of Mn4+/Mn3+, higher surface acidity, and more chemisorbed oxygen were the main causes for the presence of metal sulfates poisoning, which then resulted in the low NOx conversion at low temperature.