Here, we report a Cu<sub>2</sub>O-based photocathode applied with pulsed-laser-deposited overlayers with the structure of Ga2O3/AZO/TiO2 and 60 nm dense Pt simultaneously as hydrogen evolution catalyst and protective layer under backside illumination from ITO substrate. And by depositing thin Au nanoparticles on the bottom of Cu<sub>2</sub>O, we observe the increase of light-harvesting and the production of hot electrons from the surface plasmon resonance (SPR) effect. The modified photocathode presents a photocurrent density of - 4 mA cm<sup>-2</sup> at 0 V vs. RHE and a thermodynamically based energy conversion efficiency of 0.27% in a weak acidic electrolyte, and the stability is tremendously improved compared with other structures without the dense Pt layer. This enhancement is attributed to the formation of a p-n junction at the Cu<sub>2</sub>O and Ga<sub>2</sub>O<sub>3</sub> interface and surface stabilization by TiO<sub>2</sub> and Pt. To demonstrate an impact on practical application of the photocathode, we fabricate the bias-free solar water splitting tandem device using transparent BiVO<sub>4</sub> photoanode which exhibit a stabilized peak photocurrent density of 0.1 mA cm<sup>-2</sup> under continuous illumination.