Despite the tremendous efforts devoted to enhancing the activity of oxygen evolution reaction (OER) catalysts, there is still a huge challenge to deeply understand the electronic structure characteristics of transition metal oxide to guide the design of more active catalysts. Herein, Fe₃O₄ with oxygen vacancies (Fe₃O₄-Vac) was synthesized via Ar ion irradiation method and its OER activity was greatly improved by properly modulating the electron density around Fe atoms. The electron density of Fe₃O₄-Vac around Fe atoms increased compared to that of Fe₃O₄ according to the characterization of synchrotron-based X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS) spectra, and density functional theory (DFT) calculation. Moreover, the DFT results indicate the enhancement of the desorption of HOO* groups which significantly reduced the OER reaction barrier. Fe₃O₄-Vac catalyst shows an overpotential of 353 mV, lower than that of FeOOH (853 mV) and Fe₃O₄ (415 mV) at 10 mA cm⁻², and a low Tafel slope of 50 mV dec⁻¹ in 1 M KOH, which was even better than commercial RuO₂ at high potential. This modulation approach provides us with valuable insights for exploring efficient and robust water-splitting electrocatalysts.