In this paper, Fe3O4 magnetic nanoparticles were firstly synthesized with co-precipitation method and the Fe3O4/Ag magnetic coating modified material which has high SERS activity was further prepared by reducing sodium citrate. The structure and morphology were characterized with UV-Visible absorption spectroscopy, energy spectroscopy and transmission electron microscopy. It was found that Fe3O4/Ag material with regular morphology and the nanoparticle diameter is about 30~60 nm. Fe3O4/Ag material can easily be collected by magnets to meet the requirements of dispersion, extraction and re-enrichment under magnetic testing. Then, the theoretical and experimental Raman spectra and assignments of oxamyl on Ag surface are obtained by complexes of Oxamyl, Oxamyl-Ag and Oxamyl-Ag4 molecular structure calculation optimization based on density functional theory (DFT). Surface-enhanced Raman spectroscopy (SERS) is used to investigate the adsorption behavior and enhancement effect of oxamyl on Fe3O4/Ag and the enhancement factor of oxamyl on Fe3O4/Ag surface is calculated as 208×105. The results show that Raman spectra of the determination and the theoretical calculation have good consistency, with more Ag atoms bonded to oxamyl when closer to the NRS. Then, oxamyl is absorbed on Fe3O4/Ag by double bond side N atom and S atom. The double bond side N atom is adsorbed on the Ag surface preferentially, then the whole molecule near the Ag surface. At last the double bond side N atom and S atom adsorbed on the Ag surface together. The proposed Fe3O4/Ag magnetic nanocomposites have significant absorption enrichment and Raman enhancement effect, so Fe3O4/Ag can be used as SERS substrate to achieve rapid analysis and detection of pesticide residues oxamyl.