Transition-metal cobalt oxide (Co₃O₄) nanoparticles have been widely used as oxygen evolution reaction (OER) catalysts, but their OER catalytic performance is not ideal due to particle agglomeration. To improve the catalytic reaction performance, we report a method using g-C₃N₄ nanosheets loaded with Co₃O₄ nanoparticles. We aimed to achieve efficient OER catalytic performance by exposing more active sites in the electrochemical reaction and increase the charge transfer rate through a synergistic interaction between solid phases. Co₃O₄ nanoparticles were prepared by a hydrothermal method and high-temperature calcination. g-C₃N₄ nanosheets with a layered structure were prepared by a solid-state reaction method. Co₃O₄@g-C₃N₄ heterojunctions were prepared by simple ultrasonic chemistry and a high-temperature fusion method. The OER catalytic performance of the catalyst was tested in KOH solution (1 M). The Co₃O₄@g-C₃N₄ heterojunction catalyst had the lowest overpotential (340 mV) and lowest Tafel slope (120.92 mV dec⁻¹) when the current density was 10 mA cm⁻². The highest ECSA was 50.25 mF cm⁻², and the OER catalytic performance of the Co₃O₄@g-C₃N₄ catalyst is significantly higher than that of Co₃O₄ and g-C₃N₄. The structure and surface charge transfer pathway of g-C₃N₄ and Co₃O₄ were analyzed by DFT calculations. Spontaneous formation of an electric field at the interface promoted an asymmetric charge distribution, thus regulating the adsorption/desorption of OH⁻ during the OER. Due to the adjustment of the internal electric field and charge distribution at the heterojunction interface, the OER overpotential was reduced significantly, and the OER activity was increased significantly by Co₃O₄@g-C₃N₄. Our data provide aid rational design of high-performance electrocatalysts.