<p>Combining gold nanostructures with cuprous oxide semiconductors can integrate the plasmon resonance effects of noble metals with the visible-light-responsive properties of semiconductors, offering a novel approach to enhancing photocatalytic performance. In this study, we employed Au nanocup as the core component—a nanomaterial featuring a three-dimensional asymmetric structure that exhibits remarkable localized surface plasmon resonance and high specific surface area. By varying the amount of copper precursor, we successfully synthesized a series of Au nanocup@Cu₂O core–shell composites with different Cu₂O shell thicknesses. The influence of shell thickness on the optical properties and photocatalytic performance was systematically investigated. In the photocatalytic degradation of methyl orange under visible-light irradiation, the optimized composite demonstrated significantly enhanced activity, with the degradation efficiency being more than 12 times higher than that of pure Au nanocup or pure Cu₂O. The performance improvement is attributed to two main factors: the efficient transfer of plasmon resonance energy from the Au nanocup to the Cu₂O shell, which promotes charge generation and separation, and the formation of a heterojunction interface that facilitates charge transport. This study provides new insights for designing efficient photocatalysts and shows promising potential for environmental remediation applications.</p>