Abstract: To further enhance the light response range of SrTiO₃ and effectively suppress the recombination of its charge carriers, this study synthesized N, F-codoped SrTiO₃ (N, F-SrTiO₃) using a solvothermal method. N,F-SrTiO₃ was mixed with C₃N₄ obtained by thermal polymerization and calcined to fabricate the N,F-SrTiO₃/C₃N₄ composite. The photoelectrochemical cathodic protection performance and strengthening mechanism of this composite material were investigated in simulated seawater for 304 stainless steel (304 SS). The results demonstrated that under illumination in simulated seawater, the photocurrent density of the N, F-SrTiO₃/C₃N₄ coupled with 304 SS reached 11 μA/cm², which is 5.5 times that of pure SrTiO₃ (2 μA/cm²). Additionally, the open-circuit potential of the N, F-SrTiO₃/C₃N₄ coupled with 304 SS, was −0.52 V, which shifted negatively by 340 mV compared to the self-corrosion potential of 304 SS (−0.18 V). In comparison, the open-circuit potential of 304 SS coupled with SrTiO₃ only shifted negatively by 220 mV. The superior photoelectrochemical cathodic protection properties of N, F-SrTiO₃/C₃N₄ can be attributed to the synergistic enhancement of SrTiO₃ photoabsorption and the increased electron-hole pair separation rate achieved by the codoping of N and F as well as the composite with C₃N₄.