A visible light-driven and environmentally compatible photocatalyst is considered to be a potential application for wastewater treatment processes. In the present study, we employed carbon quantum dots (CQDs) to modify BiOCOOH with the aim of negating the UV limit of the photocatalyst, while increasing its photocatalytic activity. The structure, morphologies, and other characteristics of CQDs/BiOCOOH composites were investigated, which confirmed that CQDs were successfully coupled with BiOCOOH. Under visible light irradiation, a very low CQDs content of 2.0% weight resulted in a 4.64-fold more rapid reaction rate in the photodegradation of diclofenac (DCF) than pristine BiOCOOH. The CQDs served to enhance the visible light absorption of BiOCOOH, which significantly improved visible-light harvesting, as well as interfacial charge transfer and separation. Through a new model study it was revealed that O-2(center dot-) was predominantly responsible for DCF degradation. Based on mass spectrometry and theoretical calculations, primary intermediates were identified, and the key pathways proceeded mainly through e(-) reduction, O(2)(center dot-)attack, and (OH)-O-center dot addition reactions. The Vibrio fischeri, Desmodesmus subspicatus, and Daphnia magna were selected to evaluate the acute toxicity, which initially increased and furtherdecreased when the total organic carbon and organic chlorine compounds were reduced. The excellent dechloridation, mineralization, and detoxicity properties via the synthetic CQDs/BiOCOOH photocatalysts clearly demonstrated the potential of this strategy.