The　water　ecosystem　and　public　health　are　often　threatened　by　Arsenic　(As).　The　primary　type　of　As　in　paddy　field　and　ground　water　is　As(III),　which　is　more　toxic　and　mobile　than　As(V).　In　this　research,　biochar　prepared　from　sewage　sludge　(SBC)　through　hydrothermal　technique　was　used　to　activate　persulfate　(PS)　to　oxidate　As　(III),　making　it　less　toxic　As(V).　The　developed　SBC/PS　system　could　oxidate　As(III)　effectively　in　the　wide　pH　values　(2-10).　Excluding　the　influence　of　specific　surface　area　and　functional　groups,　the　concentration　of　environmental　persistent　free　radicals　(EPFRs)　had　a　good　linear　relationship　with　the　oxidation　of　As(III),　reaching　above　0.96.　According　to　a　mechanism　analysis,　EPFRs　on　SBC　could　transfer　electrons　to　directly　activate　PS　to　produce　SO4-center　dot,　center　dot　OH,　O-2(-)center　dot,　and　O-1(2)　to　improve　the　oxidation　of　As(III).　These　results　not　only　indicated　that　the　biochar/PS　system＇s　application　scope　can　be　successfully　increased　by　adjusting　the　preparation　conditions,　but　also　revealed　the　invaluable　role　of　EPFRs　in　As(III)　oxidation　process.