Abstract: The magnetic polystyrene nanoadsorbent material, Fe₃O₄NPs@PS, was prepared by the cross-linking polymerization method using Fe₃O₄ nanoparticles as the magnetic core, and was characterized by scanning electron microscope, transmission electron microscope, Fourier transform infrared spectrometer, and vibrating sample magnetometer. 2.00 g of sample （aquatic products） was taken and placed in a 15 mL-centrifuge tube, and 10 mL of 80% （volume fraction） methanol solution was added. The mixture was ultrasonically extracted for 4 min, and centrifuged for 5 min. The supernatant was collected, and 10.0 mg of Fe₃O₄NPs@PS was added and the mixture was ultrasonically extracted for 4 min. Fe₃O₄NPs@PS was adsorbed by a magnet, and the solution was discarded. 2.0 mL of ethyl acetate was added in twice and the mixture was shaken for elution for 3 min. The eluent was blown to near dryness by nitrogen at 40 ℃, and 200 μL of acetonitrile was added to redissolve. The solution was passed through a 0.22 μm filter membrane, and diazepam and methaqualone in the filtrate were determined by gas chromatography-mass spectrometry. As shown by the results, the synthesized Fe₃O₄NPs@PS microspheres had the core-shell structure, with the smooth surface and good superparamagnetism. Linear relationships between values of mass concentration of diazepam and methaqualone and the corresponding response values were found in the range of 1‒1 000 μg·L⁻¹, with detection limits （3S/N） of 0.30, 0.20 μg·kg⁻¹, respectively. Test for recovery was made by the standard addition method, giving results in the range of 84.9%−101%, with RSDs （n=6） of the determined values less than 7.0%. This method was applied to the analysis of 30 freshwater fish and 10 shrimp samples, and diazepam was detected in 4 freshwater fish samples, with the detected amounts in the range of 0.30−20.2 μg·kg⁻¹.