Mn-based　catalysts　are　expected　to　be　applied　for　removing　NOx　due　to　its　excellent　low-temperature　activity.　However,　the　practical　use　of　these　catalysts　is　extremely　restricted　with　the　co-poisoning　of　alkali　metal　and　SO2　in　the　flue　gas.　Here　the　MnO2/TiO2　catalyst　was　employed　to　elucidate　the　co-poisoning　mechanisms　of　K　and　SO2　for　the　low　temperature　selective　catalytic　reduction　(SCR)　of　NO.　The　physicochemical　properties　of　catalysts　under　different　toxicity　conditions　were　studied　by　experiments.　The　adsorption　of　NH3,　SO2,　NO,　and　K　on　active　component　(MnO2)　and　support　(TiO2)　was　studied　by　density　functional　theory.　This　work　unravels　a　promotion　effect　of　support　on　the　alkali　and　sulfur　resistance.　The　SO2&K　co-poisoning　catalyst　had　higher　SCR　activity　than　the　SO2-poisoned　and　K-poisoned　catalyst　alone.　For　a　single　toxic　condition:　(1)　SO2　was　preferentially　bonded　with　the　terminated　O　site　of　MnO2　inhibiting　the　dehydrogenation　of　NH3　and　redox　cycle.　(2)　The　presence　of　Lewis　base　(K　atom)　on　the　catalyst　decreased　the　binding　energy　of　a　Lewis　base　(NH3)　and　hindered　the　adsorption　of　NH3.　For　the　synergistic　effect　of　K　and　SO2,　the　majority　of　K　adsorbed　on　the　support　(TiO2)　lead　to　increase　alkalinity,　which　could　promote　the　adsorption　of　SO2　on　the　TiO2　and　reduce　the　toxicity　of　the　active　component　(MnO2).