We report a powerful approach to improve the photoelectrochemical (PEC) performance of the Si/ZnO nanowire array (NWA) photoanodes via incorporating a graphene layer. The Si/Graphene/ZnO NWAs shows the highest photocurrent, which is, respectively, 1.6 times of that for the Si/ZnO NWAs, and 6.2 times of that for the Si wafers. The introducing of ZnO NWAs and graphene greatly reduces the light reflectance, especially in the UV light region. Carrier recombination at the effective n-Si/n-ZnO junction can compensate the high valence band level of Si and thus enhances the contribution of Si to the photocurrent. The graphene interlayers offer a fast passway for the photogenerated electrons in ZnO to recombine with the photogenerated holes in Si, resulting in enhanced PEC performance of the Si/graphene/ZnO NWAs. This study demonstrates the n/graphene/n heterojunction is a promising configuration for efficient solar water splitting.