Abstract: Organic contaminants such as dyes and antibiotics have become the focus of water treatment research in recent years due to their complex composition, high toxicity, and difficulty in biodegradation. Spinel ferrite heterogeneous Fenton-like catalysts, with a chemical formula of MFe₂O₄ (MFe₂O₄, M is a divalent metallic cation or its combination, and the divalent cation is generally Ni, Zn, Mn, Co, Cu, and Mg, etc.), have attracted much attention because of their excellent structural stability and good magnetic recovery performance. However, the catalytic activity of these catalysts is not ideal and almost all the reported catalysts are synthesized by pure chemical reagents, which restrict their industrial application. Therefore, the preparation of highly efficient heterogeneous Fenton-like catalysts with low cost becomes the key to the treatment of refractory organic wastewater. In this study, copper-doped spinel ferrite (Ni, Mg, Cu)Fe₂O₄ was successfully synthesized from nickel sulfide concentrate by a coprecipitation–calcination method. The effect of copper doping concentration on the structure, micro-morphology, and catalytic performance of as-prepared samples was systematically investigated by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The optimal catalytic system was established as the photo-assisted Fenton-like catalytic system, “(Ni, Mg, Cu)Fe₂O₄ catalyst/H₂O₂/visible light”, and the enhancement mechanism of copper doping on the catalytic activity of (Mg, Ni)Fe₂O₄ was revealed. Results showed that all formed products were pure spinel ferrites under the selected synthesis conditions. With 1∶1 molar ratio of Ni to Cu, the formed (Ni, Mg, Cu)Fe₂O₄ catalyst achieved 94.5% degradation efficiency for 10-mg∙L⁻¹ RhB solution under visible light irradiation for 180 min. This observed behavior may be ascribed mainly to the increased relative contents of Fe³⁺ and Cu²⁺ ions at octahedral site. Hydroxyl radical (·OH) reaction accelerated due to increased amount of Fe³⁺ and Cu²⁺ exposed on the surface and enhanced synergetic effect between Fe³⁺ and Cu²⁺. This improved the degradation efficiency of RhB solution from 73.1% to 94.5%.