Electrochemically　synthesis　of　NH3　from　N2　using　single-atom　catalysts　(SAC)　is　one　of　the　important　strategies　for　industrial　nitrogen　fixation　and　production　of　chemical　raw　materials.　Herein,　in　the　framework　of　DFT　calculations,　the　single　transition　metal　(TM)　atoms　anchored　on　g-t-C3N4　are　systemically　studied　to　screen　the　potential　electrocatalysts　for　nitrogen　reduction　reaction　(NRR).　After　a　six-step　screening　process,　Fe@g-t-C3N4　and　Os@g-t-C3N4　SACs　are　screened　out　with　good　structural　stability,　high　selectivity　and　high　activity.　Moreover,　the　thermodynamic　and　kinetic　stability　of　Fe@g-t-C3N4　and　Os@g-t-C3N4　SACs　are　demonstrated,　suggesting　the　possibility　of　experimental　synthesis.　Furthermore,　a　series　of　electronic　properties　are　investigated　to　uncover　the　mechanism　for　the　excellent　catalytic　activity.　Generally,　three　descriptors　(ΔG*N,　φ,　ICOHP)　are　proposed　to　elucidate　the　trend　of　NRR　activity　and　to　establish　a　physical　picture　of　NRR　on　TM@g-t-C3N4　from　the　aspects　of　energy,　intrinsic　characteristics　and　electronic　properties.　This　work　provides　an　effective　and　reliable　procedure　to　screen　out　excellent　electrochemical　NRR　catalysts,　as　well　as　guidance　for　the　rational　design　of　SAC.　©　2022