Silver-deposited magnetic nanoparticles (PDA-Fe3O4@Ag) were prepared though the seed-mediated method exploiting the adsorption of silver ions by the polydopamine (PDA) which was coated on the surface of Fe3O4. The preparation process was studied using UV-Vis spectra and the prepared nanoparticles were characterized with several analytical techniques including Fourier-transform infrared (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). The catalytic action on the reduction of 4-nitrophenol and the antibacterial property of PDA-Fe3O4@Ag were studied. The result indicates that the existence of gold seeds is the key to prepare PDA-Fe3O4@Ag nanoparticles. The magnetic catalyst can be separated easily from the reaction system by applied magnetic field, and still has good catalytic performance after repeated recycling. In addition PDA-Fe3O4@Ag nanoparticles exhibit bactericidal properties, and still shows good bactericidal effect on S. aureus after recycling use for 5 times.