Silver nanoparticles (AgNPs) are one of the most effective disinfectants for eliminating pathogenic microbial contamination from water. The difficult separation of Ag-based disinfectants from water impedes their large-scale application. In this study, Ag-coated Fe₃O₄@SiO₂ magnetic composite material (MCM-Ag) is prepared by a facile route and its antibacterial activity against Escherichia coli (E. coli) is also investigated by analyzing the growth and morphology of cells after treatment. Results show that the fixed AgNPs (10–20 nm) are well dispersed on the surface of MCM-Ag. Some AgNPs are also embedded in the structure of the silica shell to avoid oxidation and release, which enhances the stability of this disinfectant. The antibacterial effect of released Ag⁺ is distinguished from that of AgNPs by monitoring E. coli growth in presence or absence of O₂. MCM-Ag with its ferromagnetic property can be repetitively used 5 times and the possible contamination of disinfectant to environment is avoided. MCM-Ag can damage the functional groups of vital enzymes and proteins of E. coli by released Ag⁺ and destroy the structure of cell membrane by the generated reactive oxygen species. It is concluded that the easily-separated MCM-Ag is an efficient and environment-friendly antibacterial agent, deserving further application.