With the rapid development of e-commerce, the number of mails has increased dramatically, and the hazardous substances in the mail have become an important criminal means for criminals, which threatens public safety and social stability. The security check of emails becomes especially important, but the conventional detection techniques can not accurately identify hazardous substances. Terahertz waves occupy the region between microwaves and the infrared. The explosives, illicit drugs and harmful biological factors concealed in the mail have a characteristic absorption spectrum in the terahertz band, and THz waves can penetrate the non-polar packaging materials commonly used in mail. Terahertz radiation also has characteristics of low energy, coherence and so on, making it possible to achieve high-sensitivity and non-destructive detection of hazardous substances by using terahertz technology. The present paper introduces the characteristics of terahertz technologies, the composition of the terahertz time-domain spectroscopy system and the Fresnel formula analytic method for obtaining optical constants. The method obtains material parameters including absorption spectra by sample transmission or reflection signals and reference signals. The terahertz characteristic absorption spectra of samples were compared with the established spectral characteristic databases of various dangerous substances to determine whether the samples were dangerous and the types of hazardous substances. The research achievements of the characteristic absorption spectra of explosives and drugs in the terahertz band and the research progress of the absorption spectra under various non-polar materials were summarized. The analytical method for obtaining the absorption spectrum is suitable for thicker samples. For the thin sample article, a P-spectrum method is also introduced, which can accurately obtain the absorption spectrum of the sample under the cover without the reference signal. In addition to directly using absorption spectroscopy for detection, in recent years, many methods for terahertz spectroscopy have been proposed, such as the spectral dynamics analysis method which can distinguish the substances with overlapping absorption frequencies well, chemometrics method which can achieve qualitative and quantitative analyses of terahertz spectra, and imaging analysis based on terahertz time domain spectroscopy which can complete the identification of hidden dangers of large areas. The feasibility of terahertz time-domain spectroscopy to identify harmful biological factors,and the characteristics of for small carrying amount of harmful biological factors were also analyzed. Meanwhile, the progress of terahertz time domain spectroscopy in improving the detection sensitivity of biological factors was summarized. Finally, the existing technique difficulties, such as limited power of terahertz, large influence by environmental factors, lack of unified standards, were discussed and the future development trend was analyzed.