Exploring　multifunctional　electrocatalysts　to　realize　efficient　hydrogen　evolution　reaction　(HER),　oxygen　evolution　reaction　(OER),　and　oxygen　reduction　reaction　(ORR)　is　urgently　desired　for　developing　novel　renewable　energy　storage　and　conversion　technologies.　However,　integrating　these　three　merits　in　one　single　catalyst　remains　a　big　challenge　due　to　the　difficulty　in　balancing　the　adsorption　strengths　of　multiple　reaction　intermediates.　Herein,　through　first-principles　calculations,　we　systematically　investigated　the　electrocatalytic　activity　of　M2B2,　M3B4,　and　M4B6　type　MBenes　(M　=　Cr,　Mn,　Fe,　Co,　and　Ni)　for　multifunctional　HER,　OER,　and　ORR.　The　results　indicate　that　most　of　the　investigated　MBenes　show　outstanding　catalytic　activity　for　HER　with　hydrogen　adsorption　Gibbs　free　energy　close　to　the　optimal　value　(0　eV).　Thereinto,　Ni2B2　and　Co3B4　MBenes　can　be　promising　multifunctional　HER/OER/ORR　electrocatalysts,　and　Fe3B4　MBene　is　expected　to　be　a　promising　bifunctional　electrocatalyst　for　HER/ORR.　Especially,　Ni2B2　MBene　is　even　better　than　the　benchmark　RuO2　catalyst　with　ultralow　low　overpotentials　of　0.26　and　0.30　V　for　OER　and　ORR,　respectively.　Then,　we　proposed　that　the　overpotentials　of　OER/ORR　can　be　well　described　by　the　varied　AGOH*　on　MBene,　which　has　been　further　illuminated　through　the　d-band　center　and　charge　transfer　analysis.　Importantly,　new　scaling　relations　between　the　adsorption　energies　of　OOH*　and　O*　on　MBenes　have　been　established,　where　AGOOH*　and　AGO*　possess　different　slopes　versus　AGOH*,　allowing　the　significantly　lower　overpotentials　of　OER　and　ORR　to　be　achieved.　This　work　provides　not　only　promising　multifunctional　HER/OER/ORR　electrocatalysts　but　also　new　scaling　relations　to　achieve　the　rational　design　of　MBene-based　electrocatalysts.