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.