Magnetic relaxation switching (MRS) sensors have shown great potential in food safety monitoring due to their high signal-to-noise ratio and simplicity, but they often suffer from insufficient sensitivity and stability due to the lack of excellent magnetic nanoprobes. Herein, dumbbell-like Au–Fe₃O₄ nanoparticles are designed as magnetic nanoprobes for developing an aflatoxin B1-MRS immunosensor. The Fe₃O₄ portion in the Au–Fe₃O₄ nanoparticles functions as the magnetic probe to provide transverse relaxation signals, while the Au segments serve as a bridge to grow Ag shell and assemble the Au–Fe₃O₄ nanoparticles, thus modulating transverse relaxation time of surrounding water molecular. The formation of Ag@Au–Fe₃O₄ is triggered by hydrogen peroxide. After degraded by horseradish peroxidase, hydrogen peroxide reduces Ag⁺ to Ag nanoparticles which assemble dispersed Au–Fe₃O₄ to aggregated Ag@Au–Fe₃O₄, thus dramatically improving the sensitivity of traditional MRS sensor. Combined with competitive immunoreaction, this Ag@Au–Fe₃O₄–MRS immunosensor can detect aflatoxin B1 with a high sensitivity (3.81 pg mL⁻¹), which improved about 21 folds and 9 folds than those of enzyme-linked immunosorbent assay and high-performance liquid chromatography (HPLC), respectively. The good consistency with HPLC in real samples detection indicates the good accuracy of this immunosensor. This Ag@Au–Fe₃O₄–MRS immunosensor offers an attractive tool for detection of harmful substances.