TiO₂ is a promising photocatalyst for overall water splitting, and its low efficiency for the hydrogen evolution reaction (HER) can be improved by loading proper cocatalysts. Accordingly, to explore the working mechanism of Ni/TiO₂ photocatalysts in the HER, we performed in-depth first-principles calculations. It was found that it is more difficult for the Ni_n cluster to undergo aggregation on the TiO₂(001) surface than the TiO₂(101) surface. The structural difference between these two surfaces determines the above-mentioned property. The adsorption of the Ni_n cluster on the TiO₂ surface is beneficial for the separation of photogenerated charges by introducing mid-gap states into the band gap and elevating the Fermi level to a higher energy region, which is favorable for the HER. An electrochemical computational method was applied to investigate the process of producing hydrogen. Our results indicate that the active site of HER in Ni_n/TiO₂ composites is the O_2c atoms binding or near to the Ni_n cluster. Thus, loading Ni_n clusters remarkably decreases the Gibbs free energy of the HER compared with clean surfaces. Our study reveals the growth behavior of the Ni cocatalyst on the anatase TiO₂ surfaces and presents a reasonable explanation for the experimental improvement in the photocatalytic activity of TiO₂ for HER upon introducing Ni cocatalysts.