Electrocatalytic　nitrogen　fixation　using　single-atom　catalysts　(SACs)　offers　a　promising　strategy　for　the　sustainable　production　of　NH3.　On　the　basis　of　density　functional　theory,　we　systematically　explored　the　potential　for　N-2　electroreduction　of　single-atom　catalysts　(SACs)　covering　V,　Nb,　and　Ta　transition　metal　(TM)　centers　supported　by　graphene　and　g-C3N4　substrates.　The　single　Nb-atom　embedded　on　g-C(3)N4　nanosheet　possesses　outstanding　nitrogen　reduction　reaction　(NRR)　catalytic　activity　and　exhibits　better　performance　than　graphene　with　a　considerably　smaller　maximum　Delta　G　value　(0.05　eV).　The　single　Nb　atom　on　g-C3N4　with　more　negative　valence　provides　structural　advantages　for　hosting　empty　d-orbitals　for　strong　N-2　and　N2H　adsorption,　as　well　as　more　single　d-electrons　to　further　promote　back-donation　to　activate　the　N　N　triple　bond.　This　work　may　be　helpful　in　developing　more　effective　TM-based　SACs　for　N-2　reduction　through　varying　substrate　effect　toward　the　same　single-atom　catalysts.