In this study, a highly efficient heterogeneous photo‐Fenton system (Fe2O3/g‐C3N4/H2O2/visible light) has been developed. The heterogeneous catalyst Fe2O3/g‐C3N4 in this system was successfully prepared by growing Fe2O3 nanoparticles on the surface of g‐C3N4. The Fe2O3 nanoparticles could achieve high dispersion on the surface of g‐C3N4 and form a heterojunction with g‐C3N4 to improve the charge separation. In addition, the combination of the Fenton's reagent Fe2O3/H2O2 and the photocatalyst g‐C3N4 greatly enhances the rate of the Fenton's reaction with the assistance of the photocatalytic process. The results showed that the Fe2O3/g‐C3N4 catalyst had a superior catalytic activity as compared with the single component of Fe2O3 or g‐C3N4 and the mechanical mixture of Fe2O3 and g‐C3N4. The catalyst prepared with 3 mL of FeCl3 aqueous solution shows the best photo‐Fenton photocatalytic efficiency with a reaction rate constant of 0.02461 mg L–1 min–1, which is about 45.4, 8.4 and 7.2 times larger than that of pure Fe2O3 (0.0005418 mg L–1 min–1), pure g‐C3N4 (0.00294 mg L–1 min–1) and the mechanically mixed Fe2O3/g‐C3N4 (0.0034 mg L–1 min–1), respectively. A possible mechanism for the visible‐light‐irradiated photo‐Fenton photocatalysis is proposed, and the Fe2O3/g‐C3N4 catalyst exhibited stable performance without obvious loss of catalytic activity after four successive runs, showing a good application prospect for the photo‐oxidative degradation of organic contaminants in wastewater.