Magnetic nanomaterials are promising heterogeneous catalysts for environmental applications. According to X-ray diffraction, Brunauer–Emmett–Teller method, scanning electron microscopy, high-resolution transmission electron microscopy, and vibrating-sample magnetometer, a kind of copper-modified nanomagnetic material (Cu-nFe3O4) was successfully prepared by a one-step synthesis method. Among them, compared with the two-step synthesis method of Cu/Fe3O4 and Cu/nFe3O4, Cu-nFe3O4 has the best effect on chlortetracycline (CTC) removal. The batch study results indicate that the maximum removal of chlortetracycline is 99.0% at a dosage = 2.0 g L–1, copper loading = 0.8 mM, and C0 = 100 mg L–1 at the optimum conditions within 90 min. The effects of humic acids (HA), NO3–, Cl–, CO32–, and PO43– on the CTC removal by Cu-nFe3O4 are also investigated. Repeated experiments were performed on the prepared Cu-nFe3O4, indicating that Cu-nFe3O4 has good recyclability. The kinetics of the Cu-nFe3O4 removal of CTC was investigated, indicating that the reaction conformed to the double constant model and the reaction is mainly dominated by a chemical reaction with physical adsorption. Finally, the mechanism of the CTC removal by Cu-nFe3O4 in a heterogeneous environment was clarified. This study aims to provide an experimental basis for the environmental application of Cu-nFe3O4.