Development of sustainable energy sources is an urgent issue to meet growing demand in world energy consumption. Photoelectrochemical cells are used to split hydrogen and oxygen from water molecules to generate chemical fuels to satisfy our ever-increasing energy demands. However, it is a major challenge to design efficient catalysts to use in the photoelectrochemical process. Recently, research has focused on carbon-based catalysts, as they are non-precious and environmentally benign. Especially, graphene possesses excellent transmittance and superior intrinsic carrier mobility, thus there have been several attempts to use graphene as a catalyst. We are the first to investigate the use of monolayer graphene as an electrocatalyst for efficient hydrogen evolution reaction (HER), employing a Si substrate as a photocathode. The understanding of the correlated interaction between solid catalysts/electrode and liquid electrolyte during HER operation is also very important for efficient hydrogen production, however, few researches have been conducted before to study the interface between carbon catalysts/semiconductor electrode and aqueous solution. Here, we have explored to develop an enabling technology and design rules for the efficient and durable catalyst/electrode system. We prepared mono-, double-, tri-, and multilayer graphene on p-Si photocathodes and investigated the layer dependence of catalytic activity for HER. Interestingly, double-layer graphene/Si exhibits noticeably improved photon-to-current efficiency and modifies the band structure of the graphene/Si photocathode. Based on in-depth electrochemical and electrical analyses, the band structure of graphene/Si was shown to result in a much lower work function than Si, accelerating the electron-to-hydrogen production potential. Specifically, plasma-treated double-layer graphene exhibited the best performance and the lowest work function. We electrochemically analyzed the mechanism at work in the graphene-assisted photoelectrode. Atomistic calculations based on the density functional theory were also carried out to more fully understand our experimental observations. We believe that investigation of the underlying mechanism in this high-performance electrode is an important contribution to efforts to develop high-efficiency metal-free carbon-based catalysts for photoelectrochemical cell hydrogen production.