CdS quantum-dots-(QDs-)dopedTiO2nanocomposites were successfully synthesized using the sol-gel technique and characterized by SEM, TEM, XRD, EDS, UV-Vis, and FS. They were then used as a new “photosensitizer” based on photodynamic therapy (PDT) for cancer-cell treatment. The photocatalytic activities of CdS-TiO2on leukemia tumors were investigated by using Cell Counting Kit-8 (CCK-8) assay. The ultrastructural morphology of treated cells was also studied by AFM. The experimental results indicated that an obvious inhibition of tumor growth would be observed in groups treated with CdS-TiO2nanocomposites, and the PDT efficiency in the presence of CdS-dopedTiO2was significantly higher than that ofTiO2, revealing that the photocatalytic activities ofTiO2could be effectively enhanced by the modification of CdS QDs. Additionally, CdS-TiO2can exhibit a very high photodynamic efficiency of 80.5% at a final concentration of 200μg/mL under visible-light irradiation. CdS-TiO2nanocomposites in this case were regarded as a promising application for cancer-cell treatment.