Abstract: The Ag@Fe₃O₄ composites with nano core-shell structure were successfully prepared by a one-pot method. The resulting Ag@Fe₃O₄ composites were characterized by TEM, XRD, UV-vis DRS and vibrating sample magnetometry (VSM). The catalytic performance and mechanism of Ag@Fe₃O₄ composites were investigated by photometrically monitoring the reduction of methyl orange in the presence of excess of NaBH₄. Furthermore, the antibacterial of Ag@Fe₃O₄ composites against Staphylococcus aureus (S.aureus) and Escherichia coli (E.coli) was studied by the paper diffusion experiment using Ag and Fe₃O₄ as the references. The results indicate that over 98% of methyl orange is catalytically degraded within 10 min. This superior catalytic activity may be resulted from transferring electron to N=N bond by Ag, thereby causing N=N fracture and generating sodium 4-aminobenzenesulfonate and p-diaminobenzene. Antibacterial experiment shows that Ag@Fe₃O₄ composites more excellent bacteriostatic activity than Ag and is more sensitive to E.coli than to S.aureus. The main reason can be ascribed to the fact that phospholipid bilayer in cell wall of E.coli is thinner than that of S.aureus.