A　simple　colorimetric　method　based　on　a　novel　oxidase-like　magnetic　composite　material　(composed　of　ferric　oxide,　amine-dendrimer　and　terminated　manganese　dioxide,　Fe3O4/PAMAM/MnO2)　was　constructed　for　the　detection　of　hydrogen　sulfide　(H2S)　in　tap　water.　The　Fe3O4/PAMAM/MnO2　composite　was　prepared　by　in-situ　reduction,　which　can　oxidize　4,3,3,5,　5-tetramethylbenzidine　(TMB)　directly　to　form　blue　oxidized　TMB　(ox　TMB)　without　the　addition　of　other　oxidants　in　2　min.　In　the　present　of　H2S,　the　corresponding　activities　of　Fe3O4/PAMAM/MnO2　composite　are　greatly　weakened　under　the　optimal　conditions.　And　the　absorbance　at　652　nm　was　linear　to　the　concentration　of　H2S　with　the　limit　of　detection　of　H2S　was　0.89　μM.　Selectivity　experiment　results　show　that　the　method　has　good　selectivity,　and　the　method　has　been　successfully　applied　to　the　determination　of　H2S　in　environmental　water　samples.　©　2022