<p>Doping has been widely explored as an effective strategy to enhance the photocatalytic performance of TiO₂ under visible light for degrading organic pollutants. However, conventional dopants derived from chemical reagents raise environmental and human safety concerns. In this study, natural down fiber (DF) was innovatively employed as a sustainable and green source of C and N, combined with copper acetate as the copper source, to fabricate C–N–Cu co-doped TiO₂ (C–N–Cu@TiO₂) via a facile one-step hydrothermal method. Comprehensive characterization revealed that, compared to pure TiO₂, the co-doped C–N–Cu@TiO₂ exhibited a narrowed band gap (from 3.11 to 2.83&#xa0;eV), reduced photoluminescence intensity, and enhanced photocurrent density, indicating improved visible-light absorption and effective suppression of electron–hole recombination. Under simulated one-sun irradiation, C–N–Cu@TiO₂ demonstrated outstanding photocatalytic activity, achieving a high degradation efficiency of 98.20% for methyl orange within 2&#xa0;h, which is competitive with many reported doped TiO₂ photocatalysts. Trapping experiments identified O₂⁻ and h⁺ as the primary active species, and a plausible photocatalytic mechanism was proposed accordingly. This work not only confirms the effectiveness of natural DF as an eco-friendly and sustainable doping source, but also encourages further exploration of biomass-derived functional materials for applications in wastewater treatment, photocatalytic hydrogen evolution, carbon dioxide reduction, and related fields.</p>