Knowledge　of　the　photocatalytic　H2-evolution　mechanism　is　of　critical　importance　for　water　splitting,　and　for　designing　active　catalysts　for　a　sustainable　energy　supply.　In　this　study,　we　prepared　plasmon　Au-modified　K-doped　defective　graphitic　carbon　nitride　(Au/KCNx)　and　then　applied　it　in　photocatalytic　hydrogen-production　tests.　The　hydrogen-production　rate　of　the　Au/KCNx　photocatalyst　(8.85　mmol　g−1　h−1)　was　found　to　be　almost　104　times　higher　than　that　of　Au/g-C3N4　(0.085　mmol　g−1　h−1),　together　with　an　apparent　quantum　efficiency　of　12.8%　at　420　nm.　It　could　significantly　improve　the　photocatalytic　activities　of　the　Au/KCNx　sample,　which　was　attributed　to　the　synergistic　effects　of　the　plasmon　effect,　potassium　doping,　and　nitrogen　vacancy.　In　addition,　the　Au/KCNx　photocatalyst　had　a　large　surface　area,　which　was　beneficial　for　photogenerated　carrier　separation　and　transfer.　The　novel　strategy　proposed　here　is　a　potential　new　method　for　the　development　of　graphitic　carbon　nitride　photocatalysts　with　obviously　enhanced　activities.　©　2023　The　Royal　Society　of　Chemistry.