<p>In this study, Tb/Mg-doped TiO₂ responsive to visible light was successfully prepared using a novel liquid-phase plasma synthesis method, and its photocatalytic activity was evaluated for use in the doxycycline decomposition reaction. The Ti-O-Tb and Ti-O-Mg bonds dispersed in the gaps of the TiO₂ lattice by doping formed a charge imbalance, which effectively separated the photogenerated electrons and holes and suppressed their recombination by generating more oxygen vacancies and Ti³⁺. The Tb/Mg-doped TiO₂ photocatalyst showed a decomposition reaction rate constant (k) that was up to twice that of bare TiO₂. When the two components were co-doped, the band gap energy and PL emission were weakened to the maximum because of the synergistic effect achieved, compared to doping with only one component. In addition, the photoelectric effect was enhanced by increasing electron-hole separation, suppressing recombination, and increasing the generation of strong oxidants. Tb/Mg-doped TiO₂ prepared by LPPSM was a reusable photocatalyst with stable photocatalytic activity, and doxycycline was estimated to be decomposed through two pathways: hydroxylation, deamination, decarboxylation, C–C bond cleavage, and a ring-opening reaction.</p>