Thiram, an organophosphate pesticide, poses a health risk due to excessive residues in agricultural products. Rapid on-site detection is essential for minimizing these risks. This study uses a modified filter paper substrate coated with silver nanoparticles (AgNPs) for fast detection of thiram via surface-enhanced Raman spectroscopy (SERS). The filter paper is modified with chloride ions to enhance AgNPs adsorption. The substrate's signal uniformity and stability were also assessed. Density functional theory (DFT) was employed to optimize thiram's molecular structure and calculate its Raman spectrum. In-situ extraction and detection of pesticide residues from apple surfaces were also explored. Results show the substrate offers excellent signal consistency and stability. A high correlation (R = 0.99) between theoretical and experimental Raman spectra was achieved, with a detection limit (LOD) as low as 10, highlighting the substrate's potential for detecting food contamination, environmental pollutants, and biomolecules.