Polymeric　carbon　nitride　(C3N4)　with　atomic　thick　layers　has　displayed　ultrashort　carrier　transfer　and　ion/molecule　migration　path　for　enhancement　of　photocatalytic　performance.　However,　rational　modulation　towards　electron　configurations　of　ultra-thin　C3N4　nanosheets　is　still　a　challenge,　because　the　existing　exfoliation　strategies　suffer　from　uncontrollable　atom-defects　and　edge　functional　groups,　leading　to　poor　solar-to-fuel　conversion　efficiency.　Herein,　a　post-redox　method　is　developed　to　form　atomic　thick　C3N4　nanosheets　with　optimized　electron　configurations　through　liquid　exfoliation　and　C-reduction.　The　as-prepared　carbon-reduced　atomic　thick　C3N4　(C-Red-AT-C3N4)　expose　more　active　sites,　greatly　enhance　charge　carrier　mobility　and　distinctly　reduce　the　optical　band　gap,　thus　leading　to　a　remarkably　improved　photocatalytic　hydrogen　evolution　rate　of　246.2　mu　mol　h(-1)　under　visible　light　irradiation　(lambda　＞　420　nm),　which　is　17-fold　higher　than　that　of　the　pristine　counterpart.　The　apparent　quantum　efficiency　reaches　18.52　%　at　420　nm　and　surpasses　most　of　existing　C3N4-based　photocatalysts.