In　recent　years,　inorganic　biomimetic　nanozymes　that　mimic　the　activity　of　natural　biological　enzymes　have　attracted　extensive　research　interest,　and　some　mimic　enzymes　have　been　successfully　applied　in　the　fields　of　biosensing,　catalysis,　and　oncotherapy.　Herein,　we　report　the　preparation　and　mechanism　study　of　a　novel　nanocomposite,　Cu2+-modified　hexagonal　boron　nitride　nanosheets-supported　subnanometer　gold　nanoparticles　(Au　NPs/Cu2+-BNNS).　Interestingly,　our　investigation　reveals　that　Cu2+-BNNS　exhibits　strong　peroxidase　mimetic　nanoenzyme　activity,　while　Au　NPs/Cu2+-BNNS　exhibits　excellent　oxidase-like　activity,　that　is,　it　can　catalyze　the　oxidation　reaction　of　the　substrate　in　the　absence　of　an　oxidant　such　as　H2O2.　For　example,　Au　NPs/Cu2+-BNNS　can　efficiently　and　selectively　oxidize　3,3′,5,5′-tetramethylbenzidine　(TMB)　and　3,3′-dimethylbiphenyl-4,4′-diamine　(OT)　coloration　without　the　presence　of　horseradish　peroxidase　(HRP)　and　H2O2.　It　is　worthy　to　note　that　AuNPs/Cu2+-BNNS-induced　TMB　coloration　only　takes　4　min　to　reach　the　platform,　while　the　conventional　HRP-H2O2　system　takes　more　than　30　min　to　reach　the　platform.　Further　mechanism　study　shows　that　the　zeta　potential,　oxidation　potential,　and　steric　hindrance　of　the　oxidative　chromogenic　substrate　determine　the　selectivity　of　oxidation　coloration,　while　the　oxidase-like　properties　of　Au　NPs/Cu2+-BNNS　are　derived　from　reactive　oxygen　species　generated　by　the　adsorbed　oxygen,　and　Cu2+　ion　can　synergistically　promote　the　oxidation　process.　Compared　with　conventional　biological　enzymes,　Au　NPs/Cu2+-BNNS　has　the　advantages　of　being　HRP　free　and　H2O2　free,　having　high　efficiency,　low　cost,　and　good　stability,　and　is　successfully　demonstrated　for　the　detection　of　carcinoembryonic　antigen　(a　universal　cancer　biomarker)　and　H2S　(the　third　gaseous　signal　molecule).　©　2019　American　Chemical　Society.