We demonstrate the fabrication of three dimensional hierarchical ZnO/Si nanowire core-shell structure, which is used as a high surface area photoelectrode in photoelectrochemical cells for solar water splitting. This structure is fabricated via a two-step solution process: fabricating vertical aligned silicon nanowires via metal assisted etching method and then growing ZnO nanowires on the silicon nanowires via hydrothermal method. Transmission electron microscopy, scanning electron microscopy, spectrometer and X-ray diffractmeter are employed to characterize the hierarchical structure. The surface area of the hierarchical structure increases significantly compared to the simplex nanowire structure, which can substantially promote the photoelectrochemical reactions. The core-shell characteristic is beneficial to the charge carrier separation and the silicon nanowire backbones can work as the charge transfer channels, which contribute to the efficiency improvement. The experimental results indicate that the photocurrent density and the maximum conversion efficiency of the hierarchical structure photoelectrode have increased obviously, confirming that this hierarchical structure is more suitable for the application of photoelectrode than the ZnO nanowires/planar silicon structure.