Extending the light absorption range of wide-band-gap semiconductor oxides is crucial to the effective solar hydrogen energy conversion. Here, we introduced surface oxygen vacancies into Mg₄Ta₂O₉ and constructed a narrow band-gap Mg₄Ta₂O_9-x photocatalyst by a simple low temperature reduction method. The as-synthesized Mg₄Ta₂O_9-x exhibits a significantly extended absorption edge in visible light range, which originates from the intermediate defect level band and electronic states created by introducing surface oxygen vacancy. With the same Pt loaded content (0.5 wt%), the Pt-loaded Mg₄Ta₂O₉, exhibits the excellent photocatalytic stability and higher hydrogen evolution activity (4.42 μmol.h^-1) than the Pt-loaded Mg₄Ta₂O₉ (0.14 [μmol.h^-1) under visible light irradiation.