The formation and stability of Nin and (NiO)n (n=1-4) clusters on the β-Ga2O3 surface have been studied by means of first-principles density functional theory calculations. It is found that the optimum interaction of the Nin and (NiO)n clusters with the surface requires different surface sites. This optimizes the formation of interfacial bonds between the atoms from clusters and the coordinatively unsaturated atoms from the surface. The stability of the adsorbed Ni clusters increases with the number of Ni atoms. In Nin/Ga2O3 system, as the Ga unoccupied states overlap with the unoccupied Ni state, the excited electrons transferred from Ga to Ni participate in proton reduction reaction. Our calculations show that (NiO)n clusters strongly adsorb on the Ga2O3 surface due to the negative adsorption energies within -1.9 eV -- -3.7 eV. For (NiO)n/Ga2O3, occupied states from the (NiO)n cluster may accept the holes from O atoms in Ga2O3 surface to take part in photocatalytic water oxidation reaction.