<p>This study focused on bismuth-based halide oxide materials with Sillén-Aurivillius phase characteristics, and a series of Cu-doped Bi₄NbO₈Cl were prepared by the molten salt method. The photocatalysts were systematically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) N₂ physisorption, UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), and photoluminescence (PL) spectroscopy to investigate crystal structures, morphological features, elemental valence states, optical absorption properties, and carrier separation efficiencies. Photocatalytic experiments were carried out to evaluate the visible-light photocatalytic activity of the as-prepared samples, using tetracycline hydrochloride (TC) as the target pollutant. The characterization results demonstrated that Cu was doped into the Bi₄NbO₈Cl lattice, which resulted in the fine-tuning of lattice parameters and the optimization of the energy band. At a nominal Cu:Nb atomic ratio of 2:8, the catalyst exhibited optimal performance, achieving a TC degradation rate of 77.0% within 2&#xa0;h under visible light irradiation. This efficiency represented 1.8 times compared to the undoped counterpart, while the apparent rate constant was 3.1 times higher than that of the pristine sample. Mechanistic investigations revealed that a narrowing of the band gap was induced by Cu doping, thereby enabling the extension of the visible-light response. Concurrently, electron–hole recombination was suppressed, and the generation of reactive species such as •OH, •O₂⁻, and h⁺ was promoted. Cycling experiments demonstrated that a degradation rate exceeding 70% was sustained by the optimal Cu-doped sample after four consecutive cycles, corroborating its structural robustness.</p>