Compared　with　the　traditional　Haber-Bosch　process,　electrocatalytic　N-2　reduction　reaction　(eNRR)　is　a　promising　strategy　for　ammonia　production　due　to　its　eco-friendly　and　energy-saving　process.　Herein,　the　feasibility　of　transition　metal　(TM)　atoms　supported　on　melon-based　carbon　nitride　(melon)　for　eNRR　was　systematically　studied　by　the　first-principles　calculations,　where　melon　is　considered　as　the　most　popular　polymeric　carbon　nitride　(PCN)　synthesized　through　the　traditional　thermally　induced　polymerization　process.　The　results　reveal　that　the　single　Mo,　Ti,　or　V　atom　anchored　on　melon　(Mo/melon,　Ti/melon,　or　V/melon)　exhibits　an　excellent　eNRR　catalytic　activity,　with　low　limiting　potential　(-0.36　V　for　Mo/melon　and　Ti/melon　and　-0.40　V　for　V/melon)　and　high　selectivity　toward　the　eNRR,　due　to　the　positive　charge　and　concentrated　spin　polarization　on　the　Mo,　Ti,　or　V　atom　as　well　as　moderate　adsorption　strength　of　eNRR　intermediates.　More　importantly,　the　NH3　desorption　is　extremely　energetically　favorable　for　Mo/melon　(0.26　eV)　and　V/melon　(0.31　eV),　endowing　their　outstanding　durability　for　eNRR.　Our　study　designed　and　screened　the　single-atom　eNRR　catalysts,　which　not　only　provides　theoretical　insight　for　the　experiments　but　also　endows　with　an　alternative　method　for　the　sustainable　synthesis　of　ammonia.