Electrocatalytic　nitrogen　reduction　reaction　(eNRR)　is　a　new　method　for　sustainable　NH3　production,　which　has　attracted　much　attention　in　recent　years.　However,　the　low　Faradaic　efficiency　due　to　the　competitive　hydrogen　evolution　reaction　(HER)　and　inert　N　equivalent　to　N　triple　bond　activation　hinders　its　practical　application.　To　find　highly　efficient　electrocatalysts　with　excellent　activity,　stability　and　selectivity,　we　have　studied　a　series　of　transition　metal　dimers　(TM2)　loaded　on　poly　triazine　imide,　(PTI)　a　crystalline　carbon　nitride,　by　density　functional　theory　calculations.　The　results　show　that　most　of　the　metal　dimers　have　good　stability.　Finally,　among　26　homonuclear　diatomic　catalysts,　Mo-2@PTI,　Re-2@PTI,　and　Pt-2@PTI　exhibit　strong　capability　for　suppressing　HER,　with　a　favorable　limiting　potential　of　-0.53,　-0.36,　and　-0.63　V,　respectively,　and　hence,　can　be　used　as　efficient　electrocatalysts　for　NRR.　In　this　study,　a　homonuclear　diatomic　eNRR　catalyst　was　designed　and　screened　to　provide　not　only　a　theoretical　basis　for　the　experiments　but　also　an　alternative　approach　for　sustainable　synthesis　of　ammonia.