<p>Rapid detection of pesticide residue using spectral technology is often hindered by the complex constituents of food matrices. Herein, a ratiometric SERS strategy is reported for detecting thiram in tea using an efficient core-shell magnetic-plasmonic substrate loaded with an internal standard (IS), Fe₃O₄@Au^4−MBN@Ag. The substrate not only utilizes its magnetic properties to enable the efficient enrichment and separation of thiram from complex matrices, but also incorporates the IS method to mitigate the issue of magnetic field non-uniformity typically arising from the aggregation of magnetic nanomaterials. Critically, high-density hotspots were engineered by optimising the gold seed loading and Ag shell thickness to achieve outstanding SERS performance. The core-shell structure shielded the Raman signal of the IS from matrix interference, thereby ensuring accurate and reliable analysis. Under optimal conditions, the method exhibited a wide linear range from 500 ng/mL to 5 ng/mL and a low detection limit of 0.15 ng/mL, which is below the EU maximum residue limit of 10 ng/mL. This work provides a robust SERS signal correction approach with promising applications in agricultural product safety.</p> Graphical abstract <p></p>