In this paper, a new ternary CuO-CeO2/g-C3N4 composite catalyst was synthesized by an impregnation method. A photo-assisted Fenton-like catalytic system was created by using visible light irradiation to activate H2O2 to remove methyl orange dye. XRD, FT-IR, SEM, EDS, and XPS were used to characterize the CuO-CeO2/g-C3N4 composite. The effect of catalyst dosage, H2O2 concentration, and pH value, and the catalyst's reusability were investigated. Under optimum working conditions, methyl orange (MO) was almost completely removed in 120 min. The catalyst operated in a wide pH range of 3-11, and inductively coupled plasma spectrometry (ICP) confirmed that it remained stable after five catalytic removal cycles. In addition, hydroxyl radicals and superoxide radicals were detected by electron paramagnetic resonance (EPR) spectroscopy and the coumarin fluorescence method, and a possible reaction mechanism was proposed.