Abstract: Fluoroquinolone (FQs) can coordinate with various metal ions in aquatic system, such interaction strongly alters their environmental behavior as well as ecological impacts. In the present study, the impacts of copper ion (Cu²⁺) on the photochemical degradation of ofloxacin (OFX), were systematically investigated. Our results revealed that OFX could complex with Cu²⁺ with a molecular ration of 1∶1 under neutral conditions. Presence of Cu²⁺ significantly inhibited OFX photodegradation, which was mainly due to the static quenching process by Cu²⁺, fluorescence quenching and laser flash photolysis experiments further confirmed this inference. High-resolution mass spectrum analysis indicated direct OFX photolysis pathways contained decarboxylation, photooxidation, piperazine ring removal and demethylation. However, demethylation products were not detected in the presence of Cu²⁺, which would reduce the potential ecotoxicity of OFX conversion process to aquatic organisms. Overall, the present study revealed the potential impact of metal ions on photochemical behavior of antibiotics, which would provide very useful information for the understanding of environmental fates of combined pollutants.