The mechanism of deactivation of MnOx/TiO2 is studied. The MnOx/TiO2 catalyst prepared by co-precipitation method has poor stability, the NO conversion decreased to about 50% after 100 h denitrification. Catalyst activity and physicochemical properties of the catalyst before and after SCR denitrification were compared and analyzed through BET, XRD, SEM, which indicated that dispersion of active sites and surface area of catalyst were decreased after 100 h denitrification. Catalyst activity and surface chemical compositions were investigated by XPS, H-2-TPR, NH3-TPD, which shown that content of Mn3+ and chemisorbed oxygen on the surface decreases. The different active sites of catalyst and adsorption characteristics of reacting gases (NH3 and NO) over catalyst before and after denitrification was investigated through in-situ DRIFT. It was found that NO is more easily adsorbed on the surface of catalyst than NH3 and form nitrate. The nitrate that did not participate in the reaction gradually accumulate on the surface of the catalyst, which inhibits the adsorption of NH3. This is the main reason for the deactivation of the catalyst.