OBJECTIVE To analyse the photodegradation product of gatifloxacin injection METHODS The photodegradation product of gatifloxacin injection was analysed by RP-HPLC/electrospray ionization mass spectrometry with a Zorbax SB-C₁₈ (4.6 mm×150 mm ID, 5μm) column. The mobile phase was composed of 3% acetic acid in acetonitrile and 3% acetic acid in water (15∶85, v/v) RESULTS The molecular weight of the photodegradation product was 355，40 less than that of gatifloxacin. The photodegradation product of gatifloxacin injection which had been stored at room temperature for 1 year after 10 days′ illumination was the same as that of the injection which had been illumination for 10 days after 14 months′ storage at room temperature CONCLUSION The structure of the photodegradation product may be 6-fluoro-1,4-dihydro-8-methoxy-7-(3-methy-1-piperazinyl)-4-oxo-3-quinolinecarboxylic acid. The gatifolxacin injection was stable protected from light.