HNS quantitative spectral analysis can not be achieved with the existing infrared Fourier transform spectrometer. As a common monomer explosive concerned, quick identification and quantitative analysis of the substance is important, so it is necessary to research and design a HNS detection system based on THz spectroscopy. Transmissive mode was used in the system to calculate the content of HNS. Electro-optic modulator module was introduced in the system, in order to achieve steady and rapid static scanning optical path. In the experiments HNS characteristic absorption peak positions were acquired. With this basis, combined air absorption characteristics of the wavelength band and spectrum data correlation expression was obtained, thus more characteristic wavelengths selected were determined. According to Beer-Lambert law, the function expression of HNS content and coefficient formula were presented. In the experiment different content of HNS sample powder was obtained with chemical configuration method with the content at the standard level. It made HNS sample powder into HNS Sample slice for all HNS samples, and it carried out HNS content testing with MINI-Z Terahertz spectroscopy and the system. Experimental results show that the results of the two methods are similar in the range of 0.10% to 50.00% for HNS content. Their error is less than 5.0% compared to the standard value, and the system has better linearity.