A　hydration-initiated　pathway　of　photocatalytic　water　oxidation　over　covalent　organic　frameworks　(COFs)　is　proposed.　The　hydration　of　water　addition　to　the　hydrazone　unit　of　hydrophilic　COF　efficiently　induces　the　initial　step　of　photocatalytic　water　oxidation.　The　subsequent　oxidative　deprotonation　and　the　coupling　of　the　bonded　hydroxyl　radicals　selectively　yield　H2O2.　Oxidative　deprotonation　is　identified　as　the　rate-determining　step,　which　can　be　facilitated　by　electron　and　proton　capture.　An　excellent　H2O2　production　rate　of　1.0　mmol　g-1　h-1　was　achieved　in　pure　water　under　atmospheric　conditions.　This　study　provides　a　mechanistic　insight　into　water　oxidation　over　COFs.