In this study, to investigate how oxygen vacancy impacts the photocatalytic performance of LaNiO₃, undoped and Sr-doped LaNiO_3−σ nanoparticles are successfully prepared by the sol-gel method. The X-ray diffractometer (XRD) results show both two samples belong to the R-3c space group of the rhombohedral system. According to the conservation of valence and the X-ray photoelectron spectroscopy (XPS) results, it is demonstrated that Sr-doping can introduce more oxygen vacancy into LaNiO_3−σ. According to photocatalytic experiments of the degradation of methyl orange (MO) solution, La_0.875Sr_0.125NiO_3−σ shows higher photocatalytic performance than undoped LaNiO_3−σ. First-principle calculation results show that the introduction of oxygen vacancy and Sr-doping can lead to the narrowing of the band gap width of LaNiO₃.