Graphitic　carbon　nitride　(g-C3N4)　is　an　actively　investigated　metal-free　photocatalyst　for　solar　energy　conversion.　However,　primary　g-C3N4　usually　exhibits　limited　utilization　of　visible　light　and　fast　combination　of　photoexcited　charge　carriers,　resulting　in　low　photocatalytic　H2　evolution　activity.　Defect-modified　g-C3N4　shows　much　enhanced　photocatalytic　H2　evolution　activity　owing　to　extended　light　absorption　as　well　as　efficient　charge　separation　and　transfer.　Here,　the　photocatalyst　simultaneously　containing　nitrogen　vacancies　and　O-doping　is　successfully　developed　by　using　a　two-step　post-synthetic　strategy　for　photocatalytic　H2　evolution,　resulting　in　a　greatly-boosted　H2　evolution　activity　(1.69　×　103　μmol　g-1　h-1)　compared　with　that　of　pristine　g-C3N4　(1.12　×　102　μmol　g-1　h-1).　It　is　believed　that　the　newly　developed　double-defect　strategy　may　open　an　avenue　toward　obtaining　molecular　level　comprehension　of　the　function　of　a　catalyst　in　photocatalytic　H2　evolution　and　can　be　extended　to　the　modification　of　other　semiconductors.　©　2018　the　Owner　Societies.