Mn/TiO2 catalyst is a promising candidate for future utilization in low-temperature NH3-SCR reaction, but its bad resistance to SO2 is still a great challenge for practical application. In this study, Eu was successfully used as the additive to improve its resistance to SO2 under SCR conditions, while the pretreatment of Mn/TiO2 and MnEu/TiO2 catalyst by SO2 + O-2 had a strong deactivation effect on them. In situ DRIFT study clarified that the deactivation of Mn/TiO2-S (SCR + SO2), Mn/TiO2-S (SO2 + O-2) and MnEu/TiO2-S (SO2 + O-2) were mainly originated from the inhibited adsorption of NH3 and NOx species, as well as the formation of a large amount surface sulfate species on them, which had a strong blacking effect on the SCR reactions over the three catalysts via both E-R and L-H routes. After the addition of Eu, SCR reaction over MnEu/TiO2 catalyst with the existence of SO2 took place through L-H pathway, accompanied by the generation of less surface sulfate species, which brought about the excellent SO2 tolerance of MnEu/TiO2 catalyst under SCR conditions.