Oxygen vacancy modulation holds great promise for enhancing the photocatalytic activity for efficient nitrogen fixation under mild conditions. In this work, the two-dimensional WO3-x nanosheets with rich oxygen vacancies were prepared using solvothermal synthesis. The WO3-x nanosheets (rich oxygen vacancies) display nice photocatalytic activity for N2 reduction to ammonia with a high yield rate of 82.41 μmol·gcat-1·h-1 under irradiation of visible light (420 nm), which is 3.59 times higher than that of the WO3-x nanoparticles (poor oxygen vacancies). Electron spin resonance (ESR), N2 adsorption-desorption isotherms, and transient photocurrent responses in the N2 or Ar atmosphere experiments proved that the rich oxygen vacancies, which are induced by the nanosheet structure, could serve as active sites for the chemisorption of N2 and facilitate the electron transfer from unsaturated sites to activated N2. Moreover, based on the analysis of banding energy, the oxygen vacancies not only boosted the ability of visible light harvesting but also elevated the defect energy level to the Fermi level, further inhibiting the defect relaxation effect. The findings offer an insight into the design of the efficient photocatalysts via structure engineering and defect engineering for photocatalytic N2 fixation.