Polymeric　carbon　nitride　(PCN,　also　called　melon)　was　a　potential　photocatalyst　for　the　CO2　reduction　reaction　(CO2RR),　but　the　conjugation　of　the　system　inhibited　the　efficiency.　By　modifying　melon　with　non-metal　elements　such　as　boron　(B),　phosphorus　(P),　oxygen　(O),　and　sulfur　(S),　we　found　that　S　(or　O)　doped　melon　(S/melon,　O/melon)　transformed　the　reactive　site　of　pyridine　nitrogen　(N)　to　carbon　(C),　which　promoted　COOH*　hydrogenation　to　CO*　and,　thus,　improved　the　CO2　reactivity　and　CO　selectivity　significantly.　In　addition,　nonadiabatic　molecular　dynamics　simulations　showed　that　the　trap　states　of　the　O/melon　and　S/melon　can　rapidly　capture　excited　electrons　to　participate　in　the　CO2RR　and　improve　the　photocatalytic　reaction　efficiency.　This　work　provided　theoretical　insight　for　the　design　of　efficient　CO2RR　photocatalysts　based　on　metal-free　materials.