Anion vacancy defect-engineered catalysts have exhibited impressive activity in the electro/photocatalytic N2 reduction reaction (NRR) because of their advantages in N2 adsorption and activation. However, most studies have concentrated on oxygen vacancy defects because of their low formation energy. Sulfur, nitrogen, and phosphorus anion vacancy defects with higher formation energies are rarely reported. Herein, utilizing a doped-oxygen-induced strategy, partially electrochemically reduced oxygen-doped Cu3P nanosheets (RO-Cu3P) with rich surface phosphorus vacancies were successfully synthesized and could be employed as highly catalytically active species for the electrochemical NRR process. It is noteworthy that both ammonia and hydrazine were detected in the NRR tests, indicating that the reduction process followed a proposed alternating pathway. An ammonia yield of 28.12 μg h-1 cm-2 with a high faradaic efficiency of 17.5% was achieved under ambient conditions for the RO-Cu3P/CFC electrode with abundant phosphorus vacancies, about 3.4 times higher than that for a prepared pure Cu3P/CFC electrode. It is believed that the RO-Cu3P species with abundant surface phosphorus vacancies are the most likely active species for NRR, as confirmed by both control experiments and density functional theory calculations. This work provides a promising strategy to design and construct efficient NRR catalysts with anion vacancy defects for good theoretical and experimental guidance.