Photodegradation of fleroxacin is investigated in different injections and solutions. After UV irradiation, fleroxacin was degraded to afford two major products in large-volume injection (specification, 200 mg:100 ml), while degraded to afford another major product in small-volume injection (specification, 200 mg:2 ml). The photodegradation products were detected and isolated by reversed-phase HPLC. Based on the spectral data (FT-IR, MS(n), TOF-MS, (1)H/(13)C, DEPT, and 2D NMR), the structures of these products were: 8-fluoro-9-(4-methyl-piperazin-1-yl)-6-oxo-2,3-dihydro-6H-1-oxa-3a-aza-phenalene-5-carboxylic acid (impurity-I); 6-fluoro-1-(2-fluoro-ethyl)-7-(2-methylamino-ethylamino)-4-oxo-1,4-dihydro-quinoline-3-carboxylic acid (impurity-II); and 6,8-difluoro-1-(2-fluoro-ethyl)-7-(2-methylamino-ethylamino)-4-oxo-1,4-dihydro-quinoline-3-carboxylic acid (impurity-III), respectively. Different photodegradation pathways of fleroxacin were proposed, which led to the different stability characteristics of fleroxacin in the injections. The fluorine atom at C8 is more photolabile in dilute injection, so defluorination and cyclization reactions are prone to take place, whereas photo irradiation only cause ring-opening oxidation reaction of piperazine side chain in concentrated injection.