Background:　Organic　dye　pollution　in　water　has　become　a　major　source　of　environmental　pollution.　Mn(III/IV)　oxides　have　attracted　a　great　deal　of　attention　to　remove　organic　dye　pollutants　due　to　their　unique　structures　and　physicochemical　properties.　Numerous　studies　have　reported　the　removal　of　dye　by　various　Mn(III/IV)　oxides　through　catalytic　degradation　and　adsorption.　The　crystalline　structures　of　manganese　oxides　and　solution　pH　may　exert　substantial　impact　on　the　removal　of　dyes.　However,　few　studies　have　focused　on　the　oxidative　degradation　of　RhB　dye　using　Mn(III/IV)　oxides　with　different　crystal　structures　during　a　spontaneous　reaction.　In　the　present　study,　three　manganese　oxides　with　different　crystal　type　(alpha-MnO2,　beta-MnO2,　and　delta-MnO2)　were　prepared　by　refluxing　process　to　decolorize　RhB　dye　in　various　pH　solutions.　Results:　The　results　showed　that　the　decolorization　efficiencies　of　RhB　for　the　three　manganese　oxides　all　increase　with　decrease　solution　pH.　alpha-MnO2　exhibited　highest　activity　and　could　efficiently　degrade　RhB　at　pH　2-6.　The　degradation　of　RhB　by　beta-MnO2　and　delta-MnO2　could　be　observed　at　pH　2-3,　and　only　little　adsorption　RhB　on　manganese　oxides　could　be　found　at　pH　4-6.　The　UPLC/MS　analysis　suggests　that　the　decolorization　of　RhB　by　manganese　oxides　consists　of　three　main　stages:　(1)　cleavage　of　the　ethyl　groups　from　RhB　molecular　to　form　Rh;　(2)　further　destruction　of　-COOH　and　-CNH2　from　Rh　to　form　the　small　molecular　substances;　(3)　mineralization　of　the　small　molecular　substances　into　CO2,　H2O,　NO3-　and　NH4+.　Conclusions:　Overall,　these　results　indicate　that　alpha-MnO2　may　be　envisaged　as　efficient　oxidants　for　the　treatment　of　organic　dye-containing　wastewater　under　acid　conditions.