Large-scale electrolysis of water to produce high-purity hydrogen is one of the effective ways to solve the energy crisis and environmental pollution problems. However, efficient, cheap and stable catalysts are one of the bottlenecks for industrial application in water splitting. Herein, a facile one-step hydrothermal process was applied to fabricate Mn-doped nickel ferrite nanosheets (Mn-NiFe₂O₄) which shown a low overpotential of 200 mV at 50 mA·cm^–2 and a small Tafel slope of 47 mV·dec^–1, together with a prominent turnover frequency (TOF) value (0.14 s^–1) and robust stability. The in-situ UV‒vis spectroscopy unveiled the surface reconstruction to generate NiOOH as active sites during oxygen evolution reaction (OER). The excellent electrocatalytic activity of Mn-NiFe₂O₄ is attributed to the vertically grown nanosheets for exposure more active sites, rich oxygen vacancies, and the hybridization between Ni 3d and O 2p orbitals caused by Mn doping. This work should provide a facile strategy by Mn-doping to simultaneously engineer oxygen vacancies and electronic structure for synergistically triggering oxygen evolution reaction.