Nanoribbon　construction　and　modification　with　functional　groups　are　important　methods　to　improve　the　performance　of　photocatalysts.　In　this　paper,　density　functional　theory　(DFT)　calculations　are　applied　to　assess　the　electron　absorption　capacity　of　different　model　structures　in　the　photocatalytic　CO2　reduction　reaction　(CO2RR),　i.e.,　melon-based　carbon　nitride　nanoribbons　(MNRs)　and　edge-modified　melon-based　carbon　nitride　nanoribbons　(X-MNRs,　X　=　NO2,　CF3,　CN,　CHO,　F,　Cl,　C　≡　CH,　OH,　SH,　CH3,　and　H).　It　is　found　that　X-MNRs　(X　=　NO2,　CN,　CHO,　CCH,　OH,　and　H)　have　a　significantly　reduced　band　gap.　Meanwhile,　the　VBM　and　CBM　are　effectively　separated　with　the　same　optical　absorption　wavelength　range,　agreeing　with　the　experimental　observations.　More　importantly,　the　Gibbs　free　energy　difference　of　the　CO2RR　rate-determining　step　is　greatly　reduced　in　MNRs,　CHO-MNRs,　CN-MNRs　etc.　with　the　formation　of　CO　or　HCOOH.　The　mechanism　investigation　indicates　that　the　materials　design　via　edge-group　modification　can　optimize　the　CO2RR　process.　©　2023　The　Royal　Society　of　Chemistry.