Abstract: In this study, the Bi₁₂O₁₇Cl₂ photocatalyst was synthesized at room temperature via a facile route using BiCl₃·5H₂O, NaOH, and CH₃CH₂OH as the raw materials. Subsequently, a series of Bi₁₂O₁₇Cl₂/TiO₂ composite photocatalysts was fabricated by mechanically mixing Bi₁₂O₁₇Cl₂ and TiO₂ at different mass ratios. A set of characterization techniques, including X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet (UV)?visible absorption spectroscopy, and transient photocurrent response measurements, were used to systematically investigate the crystal structures, morphological features, elemental chemical states, and optical properties of the as-prepared composite materials. The photocatalytic degradation efficiencies of methyl orange (MO) over pure TiO₂ and the synthesized Bi₁₂O₁₇Cl₂/TiO₂ composites were evaluated under both UV and visible light irradiations for Bi₁₂O₁₇Cl₂ mass fractions of 1%, 3%, 5%, and 7%. Under UV irradiation produced by a mercury lamp for 1.25 h, the degradation efficiencies of MO were 89% for pure TiO₂, 83% for 1% Bi₁₂O₁₇Cl₂/TiO₂, 100% for 3% Bi₁₂O₁₇Cl₂/TiO₂, 98% for 5% Bi₁₂O₁₇Cl₂/TiO₂, and 95% for 7% Bi₁₂O₁₇Cl₂/TiO₂. The photocatalytic activity of MO was in the following order: 3% Bi₁₂O₁₇Cl₂/TiO₂ > 5% Bi₁₂O₁₇Cl₂/TiO₂ > 7% Bi₁₂O₁₇Cl₂/TiO₂ > TiO₂ > 1% Bi₁₂O₁₇Cl₂/TiO₂. Thus, the highest photocatalytic activity was exhibited by the 3% composite. As the Bi₁₂O₁₇Cl₂ mass fraction increased from 1% to 7%, the MO degradation efficiency first increased and then decreased, which can be attributed to the formation of a p-n heterojunction between p-type Bi₁₂O₁₇Cl₂ and n-type TiO₂. The heterojunction not only accelerated the migration of photogenerated charge carriers but also significantly suppressed the recombination of electron-hole pairs, thereby prolonging the carrier lifetime and enhancing the photocatalytic performance of MO. When MO was irradiated by visible light produced by a 300 W xenon lamp for 2 h, the MO degradation efficiencies were 61% (TiO₂), 83% (1% composite), 90% (3% composite), 49% (5% composite), and 42% (7% composite) and were in the following order: 3% Bi₁₂O₁₇Cl₂/TiO₂ > 1% Bi₁₂O₁₇Cl₂/TiO₂ > TiO₂ > 5% Bi₁₂O₁₇Cl₂/TiO₂ > 7% Bi₁₂O₁₇Cl₂/TiO₂. As with UV light irradiation, the degradation efficiency of MO first increased and then decreased with increasing Bi₁₂O₁₇Cl₂ content, with the 3% composite exhibiting the highest visible light activity (90% MO degradation). The chemical oxygen demand (COD) removal efficiency was measured to assess the mineralization degree of MO. The COD removal efficiencies of MO were 60% (TiO₂), 63% (1% composite), 75% (3% composite), 24% (5% composite), and 11% (7% composite), which matched the order of the visible light degradation activity. The 3% Bi₁₂O₁₇Cl₂/TiO₂ composite demonstrated the optimal mineralization capability, with a COD removal rate of 75%. Mechanistic studies revealed that compared with Bi₁₂O₁₇Cl₂, TiO₂ had a more negative conduction band (CB) and a more positive valence band (VB). Under light irradiation, the photogenerated electrons in the CB of TiO₂ migrated to the CB of Bi₁₂O₁₇Cl₂, while the holes in the VB of TiO₂ transferred to the VB of Bi₁₂O₁₇Cl₂. The effective charge separation inhibited electron?hole recombination and promoted the formation of reactive oxygen species, such as ·O₂^- produced from the reaction of electrons with O₂ and h⁺, which exhibited strong oxidizing capacity that decomposed MO into inorganic products, such as CO₂, H₂O, and N₂. This study therefore provides valuable insights into the development of novel composite photocatalysts with enhanced performance, offering promising applications in the advanced treatment of organic dye pollutants and environmental protection.