Electrochemical water splitting provides a promising pathway for storing renewable energy in chemical bonds and will contribute to solving global energy and environmental issues. The whole process can be divided into two half-reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The bottleneck for the overall water electrolysis is the OER due to the higher overpotential caused by the kinetically demanding four electron/proton transfer process. Currently, IrO2-and RuO2-based catalysts have shown excellent OER activity in acidic media and have been regarded as the most promising commercial OER catalysts for proton exchange membrane (PEM) electrolysis, but their scarcity and high cost hinder their large-scale application. As a consequence, the development of cheap, highly active and stable earth-abundant transition-metal oxides metal OER electrocatalysts is critical to water electrolysis, especially Fe, Co and Ni, when alkaline water electrolysis is considered. Among them, the perovskite oxides have attracted great attention due to their low cost and flexible structure properties. Perovskite oxides have the general formula ABO3, where A is an alkaline earth metal cation (La, Ba, Sr, etc.) and B is a transition metal cation (Fe, Co, Ni, etc.). LaNiO3 (LNO), a typical perovskite oxide, has attracted much attention due to its excellent OER activity and adjustable electronic structure. Over the past decades, different strategies have been explored to enhance the electrochemical activity of LNO, including nanostructure engineering, aliovalent doping and the introduction of oxygen vacancies. Recently, Thomas and his coworkers reported that annealing LNO in H2 atmosphere can induce oxygen vacancies at the LNO surface, leading to a significant enhancement in OER activity and stability of LNO. They attributed the enhanced LNO OER performance to oxygen vacancies which promote electrical conductivity. Later, Sun et al. found that doping by Ce can significantly improve the OER performance of LaNiO3. They argue that Ce doping can introduce oxygen vacancies in LNO, which promote the conversion of Ni to a NiOOH surface phase during the OER process and thus increase the OER activity. The NiOOH surface phase being the catalytically active phase of the material. Additionally, it has been reported that LNO electrocatalysts usually undergo surface morphology and compositional changes during OER process. Liu et al. reported that the deposition of LNO particles on LNO films can significantly improve the OER activity of LNO films. They attributed the enhancement of OER activity to the fact that the particles on the surface of LNO are reconstructed to form NiOOH active species during the OER process. Later, Sun and coworkers have been directly observing the surface reconstruction process of LaNiO3 thin film during the OER by using electrochemicalscanning tunneling microscopy (EC-STM). So far, there have been many studies on the effect of oxygen vacancies on the OER performance of LNO. However, the relationship between oxygen vacancy on the surface chemical composition, electronic structure and the OER activity of LNO has not been systematically investigated and verified, especially by using a single crystal thin film model system.