Water　oxidation　reaction　(WOR)　is　the　heart　for　overall　water　splitting　owing　to　its　sluggish　kinetics.　Herein,　carbon　quantum　dots　(CQDs)　are　studied　as　co-catalyst　to　promote　WOR　by　loading　them　on　NiTiO3　(NTO)　photocatalyst.　The　performance　can　be　obtained　in　a　fold　of　7　compared　with　pristine　NTO　in　power-based　photocatalytic　system,　and　strong　stability　has　received　with　preserving　the　output　for　at　least　10　h.　The　CQDs　have　also　demonstrated　to　load　on　NTO　based　photoanode　for　WOR,　and　a　6　times　increasement　has　realized.　In-situ　characterizations　have　acquired　to　study　the　roles　of　CQDs　for　WOR　and　found　that　CQDs　can　facilitate　the　chemical　adsorption　of　water　molecules,　and　meanwhile　promote　the　formation　of　hydroxyl　radical　as　transition　states　of　WOR.　This　demonstration　presents　a　clue　to　understand　the　role　of　carbon　in　photocatalytic　system　to　promote　WOR　and　encourage　its　uses　for　advanced　photoredox　catalytic　reactions.　©　2021　Elsevier　Inc.