CuO/CeO₂ p–n heterojunction composites were synthesized using an easy microwave reflux method with homogeneous precipitation technology, and their photocatalytic performance in terms of methylene blue degradation was studied. The results showed that the degradation effect of the CuO/CeO₂ heterojunction composites was substantially stronger than that of CeO₂. The composite prepared with a Cu/Ce molar ratio of 1/3 (1/3 Cu/Ce) exhibited the highest catalytic activity, achieving a degradation efficiency of 96.2% within 180 min. The improved photocatalytic performance was attributed to the formation of a p–n heterojunction between CuO and CeO₂, which considerably promoted the separation and transport of photogenerated carriers. The band offset of the CuO/CeO₂ heterostructure was calculated; its valence band offset was 1.58 eV and conduction band offset was -0.48 eV, further demonstrating the formation of a p–n heterojunction.