How　to　improve　the　efficiency　and　selectivity　of　deep　reduction　product　during　the　process　of　CO2RR　still　remains　a　challenging　task　in　the　design　of　single　atom　catalyst　(SAC).　In　this　study,　TM/Ti2CO2　(TM　=　Sc-Ni)　with　one　surface　O　atom　being　substituted　by　either　N　(TM/Ti2CO2_N)　or　B　(TM/Ti2CO2_B)　was　investigated　via　the　density　functional　theory　with　respect　to　their　catalytic　performance　in　CO2RR.　By　comparison,　doping　boron　reduces　the　polar　covalency　of　TM-B　interaction,　whose　electronic　configuration　becomes　quite　different　from　that　of　both　TM-O　and　TM-N　interaction.　Owing　to　such　special　bonding　characteristics,　TM/Ti2CO2_B　displays　much　stronger　inclination　towards　CH4　formation　by　effectively　increasing　the　binding　strength　of　reaction　intermediates　during　CO　formation　and　breaking　the　scaling　relation　between　CO*　and　CHO*.　Furthermore,　the　work　function　and　d-band　center　were　employed　as　descriptors　to　quantify　the　catalytic　efficiency　of　different　C1　products,　from　which　the　optimum　catalysts　for　both　CO　(Mn/Ti2CO2_N,　UL　=-0.18　V)　and　CH4　(Fe/Ti2CO2_B,　UL　=-0.29　V)　have　been　determined.　Overall,　our　results　suggest　that　the　catalytic　performance　of　MXene-based　SACs　towards　different　C1　products　could　be　significantly　improved　by　introducing　the　exotic　functional　group.