Monoclinic WO3 is considered to be a promising photoanode due to high theoretical photocurrent density and quantum efficiency. However, the practical photoelectrochemical (PEC) performance is considerably inferior to the theoretical value because of severe surface charge recombination. Herein, an Ag nanoparticle (NP) modified WO3 composite photoanode was prepared via in situ cathodic reduction to boost the surface charge separation. The various PEC performance indicators such as the photocurrent, incident photon-to-current efficiency, and applied bias photon-to-current efficiency have been improved to varying degrees upon the modifications of the Ag NPs. Electrochemical impedance spectroscopy and Mott-Schottky analysis demonstrate that the Ag NPs greatly enhance the surface charge separation efficiency of WO3, thus improving the PEC performance.