Herein, a reusable and portable surface-enhanced Raman spectroscopy (SERS) sandpaper was successfully synthesized for the sensitive detection of S-fenvalerate in foods. Commercial sandpapers were decorated with Ag@SiO2@Au nanoarrays via a liquid-liquid interface self-assembly method. The capacity of sandpaper to float directly on the cyclohexane-water interface allows nanoarrays to be formed directly on it, thereby minimizing stacking issues typically associated with nanoarray assemblies and significantly enhancing the sensitivity of S-fenvalerate detection. Moreover, the SERS sandpaper was reusable and portable due to its strong adhesion of the nanoarrays. Under optimized testing conditions, the developed SERS sandpaper method was capable of detecting S-fenvalerate, demonstrating a strong linear response within a concentration range of 10–7–103 μmol/L, with a limit of detection of 1.92 × 10−8 μmol/L. The analysis of spiked food samples containing S-fenvalerate using the developed SERS sandpaper afforded excellent recoveries (92.2%−109.7%). Additionally, the SERS sandpaper was successfully applied to quantify S-fenvalerate in real food samples, with results consistent with analyses conducted using gas chromatography.