Experiments found that loading suitable cocatalysts, usually in the form of metal and metal oxide nanoparticles, on the semiconductor surface can remarkably increase the photocatalytic activity of water-splitting reaction. To get insight into the mechanism of experimental observations, we took Ni-NiO/beta-Ga2O3 photocatalytic system as a model and performed detailed density functional theory calculations. Electrochemical computational methods are used to investigate the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Our results show that in Ni-n/Ga2O3 (n = 2, 3 and 4) systems, the reaction sites of HER are on Ni-n clusters. Loading Ni-n clusters on Ga2O3 surface importantly reduces the reaction free energy of HER. On the clean Ga2O3 surface, water dissociation is energetically unfavorable. After attaching NinOn (n = 2 and 4) clusters to the surface, water decomposition becomes thermodynamically favorable due to the unsaturated coordination of Ni and O atoms in clusters. The favorable reaction sites of OER focus on NinOn clusters. The rate-determining step of OER can be changed by adsorbing NinOn clusters. Notable reduction of overpotential (0.77-0.87 V) for OER on NinOn/Ga2O3 is found compared with that on clean Ga2O3 surface, which reasonably explains the experimental observation on significant enhancement of activity for generating oxygen after loading NiOx cocatalysts. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.