Photocatalysis technology is an effective means to address the issue of energy and environmental pollution. In this paper, defect engineering is introduced to couple g-C₃N₅, which further improves the photocatalytic oxidation of mercury by Bi₅O₇I. The experimental results show that defect engineering can not only regulate superoxide radicals as intermediates but also act as active sites to accelerate carrier transport, thereby increasing the photocatalytic mercury removal efficiency to 96.2%. The chemical reaction rate of g-C₃N₅/Bi₅O₇I heterojunction photocatalyst was 8.01 times that of pure g-C₃N₅ and 4.58 times that of pure Bi₅O₇I. Through XPS and active radical capture test experiments, it was found that ·O₂⁻, h⁺, and ·OH play a major role in mercury removal experiments. Finally, a collaborative strategy of vacancy structure and built-in electric field is proposed, which improves the charge transport efficiency of g-C₃N₅/Bi₅O₇I Z-scheme heterojunction interface. Our work provides theoretical support for the application of g-C₃N₅ and its composites in the field of flue gas mercury removal.