A magnetic photocatalyst was synthesized by direct growth of CdS nanoparticles on the surface of magnetic fly ash cenospheres (MFACs) through chemical deposition. The photodegradation of danofloxacin mesylate (76.2%) demonstrated that CdS/MFACs exhibited higher photocatalytic activity than that of pure CdS (71.5%) under visible light irradiation. Moreover, the as-prepared photocatalyst was relatively stable and displayed good reusability after 5 cycles. A reasonable photocatalytic mechanism for CdS/MFACs was verified by reactive species trapping, and further confirmed by ESR and a radical scavenging species, DMPO. Furthermore, qualitative hydroxyl radical analysis showed that ˙OH was present during the photocatalytic degradation process and was only generated from the photogenerated electrons in the conduction band of CdS/MFACs. In addition, the experimental results showed that the photodegradation of danofloxacin mesylate is associated with ˙O₂⁻ and photogenerated holes. Finally, the photocatalysis degradation reaction kinetics and the mechanism of photodegradation of danofloxacin mesylate were also discussed.