Photocatalytic water oxidation suffers from sluggish kinetics and remains the bottleneck for water splitting. Here, using CeO2 nanorods as model photocatalyst we studied the critical role of oxygen vacancies in photo catalytic water oxidation. First CeO2 nanorods with similar morphology but different concentration of oxygen vacancies were fabricated by one-step hydrothermal method with in-situ reducing treatment. The optical absorption, charge transfer efficiency, and photocatalytic activity in oxygen generation were found closely dependent on the concentration of oxygen vacancies. Then density functional theory calculations were conducted to unveil the role of oxygen vacancies and understand the water oxidation mechanism. It was found the presence of oxygen vacancies narrows the bandgap and modulates the electronic structure for accelerating the charge transfer, in good agreement with the experimental observations. The overall oxygen generation pathway was screened and the oxygen vacancies were found to lower the barrier energy for the rate limiting step of O-O bond formation and restrain the reverse reaction of O and H, thus the O-2 generation kinetics on oxygen-defective CeO2 are improved significantly. This study provides in-depth understanding on the critical role of oxygen vacancies in photocatalytic water oxidation and is helpful for designing highly efficient photocatalyst to overcome the bottleneck of water splitting.