In this paper, L-cysteine (L-Cys) modified Fe3O4-coated titanium dioxide (Fe3O4@TiO2/L-Cys) composite nanoparticles were prepared by coprecipitation method.The composite nanoparticles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The effects of composite nanoparticles on PDT inactivation of HL60 cells in vitro were discussed.The inactivation mechanism of PDT was preliminarily explored. The results showed that Fe3O4@TiO2/L-Cys composite nanoparticles had high dispersibility, good biocompatibility, lower dark toxicity to cells, and could effectively enhance the targeting and improve the inactivation efficiency of PDT. When the irradiation dose was 18 J/cm2 at 410 nm, the overall PDT efficiency was the highest when the ratio of titanium dioxide to Fe3O4 was 1∶3. The PDT inactivation efficiency of Fe3O4@TiO2/L-Cys was 69.36%.