The　solar　conversion　of　nitrogen　to　ammonia　is　a　green,　sustainable,　and　promising　way　to　fix　nitrogen.　However,　designing　a　photocatalyst　with　high　activity,　selectivity,　and　stability　for　the　N2　reduction　reaction　(NRR)　is　challenging　because　of　the　slow　inert　activation　of　N2　and　competitive　hydrogen　evolution　reaction　(HER).　Herein,　a　single　metal　site　anchored　to　a　triazine-based　covalent　organic　framework　(Tr-COF)　backbone　(named　TM@Tr-COF;　TM　=　Fe,　Co　and　Ni)　is　fabricated　for　high-performance　catalytic　N2　reduction.　Density　functional　theory　calculations　show　that　the　Fe@Tr-COF,　Co@Tr-COF　and　Ni@Tr-COF　can　effectively　activate　N2　and　reduce　it　to　NH3　via　the　electron　＂acceptance-donation＂　process.　Meanwhile,　the　NRR　occurs　via　the　enzymatic　pathway　on　the　Fe@Tr-COF,　Co@Tr-COF　and　Ni@Tr-COF　with　a　limiting　potential　of　0.38　V,　0.58　V　and　0.54　V,　respectively.　Furthermore,　the　anchoring　of　Fe/Co/Ni　on　the　Tr-COF　allows　the　Tr-COF　to　be　adjusted　to　the　appropriate　edge　position　and　visible　light-absorption　region,　indicating　that　the　system　may　be　a　promising　and　efficient　photocatalyst.　Compared　with　other　competitive　reactions,　the　system　exhibits　high　selectivity　for　NH3　production　and　inhibits　competitive　HERs.　These　findings　have　considerable　implications　for　innovatively　designing　highly　active　single-atom　catalysts　supported　by　COFs.