Terahertz (THz) radiation is an electromagnetic radiation with a frequency that lies between the microwave and infrared regions of a spectrum. It has various unique properties, including transmission, transience, broadband, coherence, and low energy and can reveal the weak interactions between molecules. The THz application became possible due to the development of THz sources and detectors. Accordingly, with the development of machine learning, THz applications are being expanded to test many commonly used nonpolar dielectric materials and mixtures with a similar fingerprint. In this work, we introduce the theory of the optical system and the analytical method of a spectrum. We discuss the advanced applications of the THz technology in agriculture, including agricultural bio-molecular material detection, crop physiology inspection, soil detection, pesticide and antibiotic residue detection, and agricultural product and seed quality detection. In summary, we outlook the prospect of the development of terahertz time domain spectroscopy to promote its application in agriculture.