In the early stage of the investigation, the rapid detection and examination of suspicious stains at crime-related scenes are crucial. Based on the frequent use of herbicides to poison humans and livestock in grassroots public security cases in recent years, easily purchased and common herbicides are combined with white paper, fabric, and corn. All samples are scanned using a pulsed terahertz time-domain imaging system, and the reflection imaging images in the terahertz band of all samples and frequency domain spectra of some samples are obtained. The characteristics and differences between these images and spectra are analyzed and summarized. ImageJ software and the Otsu algorithm are used to process some images, and median filtering, histogram equalization, and sharpening are used to enhance the imaging results. This study validates the feasibility of using pulsed terahertz time-domain imaging technology combined with machine learning to detect suspicious stains non-destructively and effectively in grassroots cases.