Oxygen vacancies in oxygen evolution cocatalysts (OECs) can significantly improve the photoelectrochemical (PEC) water splitting performance of photoanodes. However, OECs with abundant oxygen vacancies have a poor stability when exposing to the highly-oxidizing photogenerated holes. Herein, we partly fill oxygen vacancies in a MnCo₂O_x OEC with N atoms by a combined electrodeposition and sol-gel method, which dramatically improves both photocurrent density and stability of a BiVO₄ photoanode. The optimized N filled oxygen vacancy-rich MnCo₂O_x/BiVO₄ photoanode (3 at.% of N) exhibits an outstanding photocurrent density of 6.5 mA·cm⁻² at 1.23 V_RHE under AM 1.5 G illumination (100 mW·cm⁻²), and an excellent stability of over 150 h. Systematic characterizations and theoretical calculations demonstrate that N atoms stabilize the defect structure and modulate the surface electron distribution, which significantly enhances the stability and further increases the photocurrent density. Meanwhile, other heteroatoms such as carbon, phosphorus, and sulfur are confirmed to have similar effects on improving PEC water splitting performance of photoanodes.