Inorganic photocatalysts have been regarded as a promising candidate in the domain of tumor photody-namic therapy (PDT) due to their inspirational photocat-alytic activity.In this study,a Bi4O5Br2 photocatalyst was synthesized and it exhibited effective photo-killing activity of A549 cells (a human lung carcinoma epithelial cell line)in vitro.On this basis,we modified Bi4O5Br2 with carbon quantum dots (CQDs) via a hydrolysis method at room temperature,which resulted in an improved photo-killing effect of Bi4O5Br2 to A549 cells.The samples and the interaction between samples and cells were fully charac-terized.It has been found that the loading of CQDs on Bi4O5Br2 can reduce the hydration ratio,increase the cel-lular uptake and improve the photogenerated reactive oxygen species (ROS) as compared with pristine Bi4O5Br2.Electron spin resonance (ESR) analysis and radical-trap-ping experiments manifested that the ROS contributed to PDT may be ·O2-and ·OH.This study may provide a useful strategy to ameliorate the penetrability,cell com-patibility and PDT effect upon cancer cells of other inor-ganic photocatalysts.