MnCeOx/TiO2 has been widely used in selective catalytic reduction (SCR) of NOx at low temperature. However, it is often poisoned in the presence of water vapor and sulfur dioxide. In this work, the pro-motion mechanism of Ba modification was investigated. Results show that the doped BaO reacts with CeO2 and forms BaCeO3. This unique perovskite structure of BaCeO3 significantly enhances NO oxidation and NH3 activation of MnCeOx/TiO2 catalyst so that the NO conversion and the resistances to SO2 improve. It is found that Ba species obviously promotes the NO adsorption ability and improve the redox properties of MnCeOx/TiO2 catalyst. While the acid properties of the catalyst are inhibited by Ba modi-fication and among which Lewis acid sites are dominant for both MnCeOx/TiO2 and MnCe(Ba)Ox/TiO2 catalysts. Furthermore, in situ DRIFT experiments reveal that the NO reduction upon MnCeOx/TiO2 and MnCe(Ba)Ox/TiO2 catalysts follows both E-R and L-H mechanisms, in which L-H is preferred. Ba species enhances the formation of active nitrate species, which accelerates the NO reduction through L-H mechanism. It is interesting that although Ba species weakens the NH3 adsorption, it induces the ammonia conversion to coordination ammonia, which in turn accelerates the catalytic reaction.