Isocarbophos is a highly effective broad-spectrum pesticide that is often used in vegetable pest control. However, it is highly toxic and produces residues that pose a severe risk to human health. Therefore, this paper proposes an approach based on metamaterial terahertz technology for detecting isocarbophos at low mass concentrations in vegetables. According to the split-ring resonator principle, a high-performance metamaterial is designed with a quality factor (Q) of 65. By testing the terahertz spectra of different mass concentrations of isocarbophos in cabbage leaves containing metamaterials, an exponential fitting model between the frequency shift of the resonance peak and the mass concentration is established. The determination coefficient R² is 0.99765, and the lowest mass concentration detected in the experiment is 0.00087 mg/L. Compared with traditional methods, metamaterial-based terahertz technology has a lower detection limit and simplifies the pretreatment process. The proposed method determines low mass concentrations of isocarbophos in vegetables rapidly and effectively.