As a type of nitromethylene nicotine insecticide, midacloprid has been widely used in agricultural production due to its wide-spectrum, high efficiency and low toxicity. However, its excessive residue poses a threat to human health. We first analyze the terahertz transmission spectrum of the metamaterial structure and explains the reason for the formation of the resonance frequency. Secondly, both the metamaterial structure and the silica substratecoated with 500 mg·L-1 imidacloprid solution were measured, respectively, and the influence of the silica substrate was excluded. Next, we prepared imidacloprid solutions with three gradients and 15 concentrations, which were respectively: 100~500, 10～50, 1～5 mg·L-1, and then the terahertz time-domain spectra of these imidacloprid films sprayed on the metamaterial were measured. According to the different peak frequency redshifts of terahertz transmission spectrum, the identification of different solution concentrations was realized, and the functional relationship between the peak frequency redshifts and the concentration of imidacloprid was established. The experimental results show that the detectable concentration of imidacloprid film can be as low as 1 mg·L-1 by THz spectroscopy, along with the increase of imidacloprid concentrations, and peak frequency of the red shift curve increased. Lastly, the refractive index of different concentration imidacloprid solution was substituted into CST software for simulation, and results demonstrated that the transmission curves of different concentration imidacloprid had different redshifts. The above experimental and simulation results show that the peak frequency modulation can be used to detect the low concentration imidacloprid by using THz-TDS with metamaterial. This study provides a new method for the detection of pesticide residues in the food matrix.