A novel gap-plasmon of Fe_3O_4@Ag core-shell nanoparticles for surface enhanced fluorescence detection of Rhodamine B(RB) was developed. Fe_3O_4@Ag core-shell nanostructures with Ag shell and Fe_3O_4 core were synthetized by self-assembled method with the assistance of 3-mercaptopropyl trimethoxy silane(MPTS). To study the RB fluorescence enhanced by gap-plasmon, the fluorescence properties of RB on the substrates with different nanogap densities were systematically investigated, and the results showed that the fluorescence intensity of RB on Fe_3O_4@Ag core-shell NPs substrate was much stronger than that on bare glass substrate, and the fluorescence intensity was further improved by using multilayer Fe_3O_4@Ag core-shell NPs substrate which had higher nanogap density. Different from the mechanism that is based on the maximum overlap of the surface plasmon resonance(SPR) band and emission band, the mechanism of the fluorescence enhancement in our work is based on the localized surface plasmon(LSP) and the gap plasmon near-field coupling with the Fe_3O_4@Ag core-shell NPs. Besides, the detection limit obtained was as low as 1×10~(-7) mol/L, and the Fe_3O_4@Ag core-shell NPs substrate had high selectivity for RB fluorophores. It was demonstrated that the Fe_3O_4@Ag core-shell NPs substrate had activity, good stability, and selectivity for fluorescence detection of RB. And the detection of RB by the surface plasmon enhanced fluorescence was more convenient and rapid than the traditional detection methods in previous works. A novel gap-plasmon of Fe_3O_4 @ Ag core-shell nanoparticles for surface enhanced fluorescence detection of Rhodamine B (RB) was developed. Fe_3O_4 @ Ag core-shell nanostructures with Ag shell and Fe_3O_4 core were synthetized by self-assembled method with the assistance of the 3-mercaptopropyl trimethoxy silane (MPTS). To study the RB fluorescence enhanced by gap-plasmon, the fluorescence properties of RB on the substrates with different nanogap densities were systematically investigated, and the results showed that the fluorescence intensity of RB on Fe_3O_4 @ Ag core-shell NPs substrate was much stronger than that on bare glass substrate, and the fluorescence intensity was further improved by using multilayer Fe_3O_4 @ Ag core-shell NPs substrate which had higher nanogap density. Different from the mechanism that is based on the maximum overlap of the plasmonic resonance (SPR) band and emission band, the mechanism of the fluorescence enhancement in our work is based on the localized surface plasmon (LSP) and the gap plasmon near-field coupling with the Fe_3O_4 @ Ag core-shell NPs. Besides, the detection limit obtained was as low as 1 × 10 ~ (-7) mol / L, and the Fe_3O_4 @ Ag core -shell NPs substrate had high selectivity for RB fluorophores. It was demonstrated that the Fe_3O_4 @ Ag core-shell NPs substrate had activity, good stability, and selectivity for fluorescence detection of RB. And the detection of RB by the surface plasmon enhanced fluorescence was more convenient and rapid than the traditional detection methods in previous works.