Objective To prepare a granulocyte targeting-mediated magnetic-fluorescent nanoprobe for detecting prostate cancer PC3 cells in vitro. Methods The nanometer materials with magnetic and red fluorescence, which were prepared using Fe₃O₄ as the core, and SiO₂ and rhodamine isothiocyanate as the shell, were mixed with normal human peripheral blood granulocytes in different proportions, and co-incubated for different periods to examine the toxicity of nanometer materials to granulocytes. The best proportion was selected to combine the nanometer materials and granulocytes in vitro. Finally we obtained the granulocyte targeting-mediated magnetic-fluorescent nanoprobes. We mixed PC3 cells and normal human whole blood cells in different proportions, added the nanoprobes, and then observed the targeting situation of the nanoprobes under a fluorescence microscope. Results The nanoprobe had no obvious influence on the survival rate of granulocytes at different concentrations and action times set in this study. The nanoprobes were enriched around the PC3 cells with a "petal-like" structure, but the peripheral blood cells were not enriched by probes. Conclusion The magnetic-fluorescent nanometer materials prepared in this study have no toxicity to granulocytes, and it can effectively detect tumor cells by the biological targeting effect of granulocytes on tumor cells.