Nickel-based materials have been used in electrochemical water splitting and as a co-catalyst to effectively improve photocatalytic activity. However, the mechanism of the catalytic effect on hydrogen evolution of NiO and Ni(OH)2 needs further investigation because of the high risk in self-reduction with photo-induced electrons. In this work, the self-reduction of Ni2+ was confirmed during the photocatalytic H2 evolution in the CdS/Ni(OH)2 hybrid materials. Meanwhile, in situ formed metallic Ni plays an important role in the increase in the catalytic activity. Fortunately, only a few photo-induced electrons participate in the reduction of Ni2+ in CdS/Ni(OH)2 hybrid materials. The existence of metallic Ni would prevent more Ni2+ from self-reduction. The synergistic effect of Ni(OH)2 and metallic Ni contributes to the improvement of H2 evolution of CdS nanorods.