BiOCl has wide application in the photocatalytic nitrogen fixation field. However, rapid recombination of photogenerated electron-hole pairs limited its development. In this paper, a novel RuO2/BiOCl composite catalyst with abundant oxygen vacancies was firstly obtained by a simple hydrolysis method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to analyze the chemical composition and morphology of the composite photocatalyst. In addition, the optical properties of RuO2/BiOCl catalyst were tested by UV-Vis diffuse reflectometry spectroscopy (UV-Vis DRS) and photoluminescence spectroscopy (PL). The synthetic process of photocatalyst was monitored by EPR characterization, indicating that interaction between RuO2 and BiOCl during the synthesis reaction leads to numerous oxygen defects. The photocatalytic nitrogen fixation performance was evaluated by a 300 W Xe lamp as a simulated sunlight source. The results reveal that the composite catalyst exhibits better nitrogen fixation activity than pure BiOCl, with optimum activity of 131.9 μmol/L after 1 h irradiation when the RuO2 loading reaches 0.2% (mass fraction), which is 3.5 times of the pure BiOCl. Finally, the underlying photocatalytic reaction mechanism of the catalyst was explored. The improved nitrogen fixation activity may be attributed to the fact that RuO2NCs could accelerate the transfer and consumption of holes. Besides, the oxygen defects are conducive to the adsorption and activation of nitrogen. The results clarify that the oxygen defects led by the inter-component contact can also act as N2 catalytic activity sites, resulting in higher nitrogen fixation activity. This work provides ideas for the preparation of photocatalysts with higher nitrogen fixation activity.