On　the　basis　of　the　electron　＇acceptance-donation＇concept,　a　boron　decorated　melon-based　carbon　nitride　(CN)　is　studied　as　a　metal-free　photocatalyst　to　efficiently　reduce　N2　to　NH3　under　visible　light　irradiation.　The　results　revealed　that　a　boron-interstitial　(Bint)-decorated　melon-based　CN　has　an　outstanding　N2　reduction　capacity　through　the　enzymatic　mechanism　with　a　rather　low　overpotential　(0.32　V).　The　excellent　efficiency　and　selectivity　of　Bint-decorated　melon-based　CN　in　N2　reduction　reaction　(NRR)　are　attributed　to　the　concentrated　spin　polarization　on　the　B　atom,　the　significant　enhancement　of　visible　and　infrared　light　absorption,　and　the　effective　inhibition　of　the　competitive　hydrogen　evolution　reaction　(HER).　Importantly,　B-doped　melon-based　CN　has　been　successfully　synthesized　in　the　experiments,　so　obtaining　Bint-decorated　melon　is　promising,　while　proton　transfer　from　the　-NH2　group　in　CN　to　the　B　atom　surely　will　affect　the　functionality　of　the　catalyst　through　deactivation　of　the　N2　adsorption　site.　Our　study　provides　a　novel　single　atom　metal-free　photocatalyst　with　high　efficiency　for　NRR,　which　is　conducive　to　the　sustainable　synthesis　of　ammonia.　This　journal　is　©　the　Owner　Societies.