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/cm2 at 410 nm, the overall PDT efficiency was the highest when the ratio of titanium dioxide to Fe₃O₄@TiO₂was 1:3. The PDT inactivation efficiency of Fe₃O₄@TiO₂/L-Cys was 69.36%.