Constructing a highly efficient and stable photocatalyst is a promising strategy to protect the environment. In this work, we creatively synthesized a novel CdS/Bi4NbO8Cl heterostructured photocatalyst by in situ hydrothermal method. By using ciprofloxacin as target antibiotic residue pollutant, it is found that the CdS/Bi4NbO8Cl shows improved photocatalytic efficiency compared to bare CdS or Bi4NbO8Cl, and the photocatalytic activity of CdS/Bi4NbO8Cl with the molar ratio of 0.3 is the highest, reaching about 90%. X-ray diffraction, scanning electron microscope, and transmission electron microscope were employed to study the composition, morphology, and structure of the composite photocatalysts, while UV-vis diffuse reflection (UV-vis DRS), photoluminescence, transient photocurrent response, electrochemical impedance spectroscopy, and Mott-Schottky plots together show that the significant improvement in photocatalytic activity is due to synergistic effects of enhanced light harvesting and separation of photo-induced charges carriers, caused by the intimate contact between CdS and Bi4NbO8Cl. It is expected that the facile, low-cost method and the excellent performance of the synthesized photocatalyst in this paper can contribute to the management of water pollution.