The unabated release of fluoroquinolone compounds into surface water led to environmental problems. In this contribution, the heterogeneous photocatalytic technology was employed utilizing carbon quantum dot CQDs/Bi5Nb3O15 nanocomposites for degrading the veterinary pharmaceutical sarafloxacin as a model pollutant. A simple hydrothermal method was used to synthesize CQDs/Bi5Nb3O15 nanocomposites with different CQDs amount (1-4 wt%). X-ray diffraction and Fourier transform infrared spectra revealed the formation of CQDs/Bi5Nb3O15 nanocomposites. UV-vis spectra of CQDs/Bi5Nb3O15 nanocomposites displayed a marked absorption enhancement in the range of 400-700 nm. Transmission electron microscopy images demonstrated the as-prepared CQDs having small particle size were successfully coupled with Bi5Nb3O15. X-ray photoelectron spectra indicated the presence of C, O, Bi and Nb in CQDs/Bi5Nb3O15 nanocomposites. Brunauer-Emmett-Teller measurements confirmed that the surface area of 3 wt% CQDs/Bi5Nb3O15 nanocomposites was higher than that of Bi5Nb3O15. The photocatalytic decomposition of sarafloxacin was investigated using these novel nanocomposites under irradiation of 250W Xe lamp. The prepared 3 wt% CQDs/Bi5Nb3O15 nanocomposites exhibited better degradation efficiency than pure Bi5Nb3O15 nanoparticles. In addition, the degradation products were identified by UHPLC/MS/MS and possible degradation pathways were proposed. Four new intermediates were found, featuring a series of steps including cleavage of piperazine ring and oxidation.