Among many candidates for photoanode materials of photoelectrochemical (PEC) cell, nanostructured tungsten trioxide (WO3) is regarded as one of the most promising materials due to its superior electrical properties and adequate bandgap (~2.8eV) and band edge position. WO3 nanoflakes(WO3 NFs), which have merits on its high surface area and crystallinity, have been actively studied for this manner but solar-to-hydrogen (STH) efficiency of WO3 NFs based photoanode is still not sufficient both in light absorption and charge separation. Plasmon-induced enhancement using Au nanoparticles is excellent approach for both the efficiency of light absorption and charge separation of WO3. However, it still needs optimization on its amount, shape, coverage, and e.t.c. Here, we synthesized WO3 NFs by solvothermal growth and decorated gold nanoparticles on these nanoflakes by e-beam evaporation and rapid thermal annealing (RTA) process in a row. By this process, a large-area AuNPs/WO3 nanocomposite structure with various size, interparticle distance, and coverage of AuNPs were fabricated. These AuNPs/WO3 NFs type photoanode achieve high light absorption both in UV and visible range and consequently higher photocurrent density. The optimized AuNPs/WO3 nanocomposite photoanode exhibits 1.01mA/cm2 of photocurrent density, which is increased to 19.8% compared with bare WO3 nanoflakes. Field emission-scanning electron microscope(FE-SEM), X-ray Diffraction(XRD), UV-vis spectrometer analysis were measured to analyze the morphology and crystallinity and relationship between structure and PEC performance.