First-principles calculations are used to study alloy effects on configurations, stability and water adsorption of PtRu_n-1(n=2-14) and H₂O-PtRu_n-1 (n=2-14) systems. It shows that substitution energy of a Pt atom to a Ru atom is low which manifests that Pt is easy to form alloy with Ru clusters. Compared with pure Ru_n cluster, alloy effects enhance adsorption energy of H₂O molecule on PtRu_n cluster, and H₂O molecule is not easy to release in molecular form from PtRu_n cluster. Considering van de Waals force, adsorption energy of water on PtRu₇ increased and dissociation barrier decreased, making it possible to split water on PtRu₇. In conclusion, PtRu_n is suitable to be catalyst for spliting water and producing hydrogen.