Single-atom　catalysts　(SACs)　are　emerging　as　promising　catalysts　in　the　field　of　the　electrocatalytic　CO2　reduction　reaction　(CO2RR).　Herein,　a　series　of　3d　to　5d　transition　metal　atoms　supported　on　triazinebased　graphite　carbon　nitride　(TM@TGCN)　as　a　CO2　reduction　catalyst　are　studied　via　density　functional　theory　computations.　Eventually,　four　TM@TGCN　catalysts　(TM　=　Ni,　Rh,　Os,　and　Ir)　are　selected　using　a　five-step　screening　method,　in　which　Rh@TGCN　and　Ni@TGCN　show　a　low　limiting　potential　of　-0.48　and　-0.58　V,　respectively,　for　reducing　CO2　to　CH4.　The　activity　mechanism　shows　that　the　catalysts　with　a　negative　d-band　center　and　optimal　positive　charge　can　improve　the　CO2RR　performance.　Our　study　provides　theoretical　guidance　for　the　rational　design　of　highly　active　and　selective　catalysts.