The serious surface charge recombination, slow charge transfer kinetics and the poor photoelectrochemical (PEC) stability are the main challenges for PEC water-splitting performance of semiconducting metal oxide. Designing photoelectrode by incorporation of photo-active nanomaterial and suitable passivation layer is an emerging strategy to overcome limitations mentioned above. Here, we investigate the influence of 2D graphene layer on the PEC performance of WO₃ thin film according to the number of graphene layers and elucidate interfacial electronic properties and transport mechanisms. Monolyaer-graphene(1LG)/WO₃ Schottky junction promotes hole transfer from semiconductor to electrolyte suppressing recombination of charge carrier and self-oxidation of photoelectrode. On the other hand, multilayer-graphene(NLG)/WO₃ exhibits enhanced electrocatalytic activities toward hydrogen evolution reaction due to electrochemical properties of graphene edges and reduction of Schottky barrier height at NLG/WO₃ interface. Additionally, such extremely thin 1LG and NLG protect the photoelectrode from photo-corrosion without disturbing the light absorption. These results can provide a new route for advanced designs of nanomaterial-based PEC devices.