Fluid inclusion is one of the most important methods to study the evolution of mineral crystals, which can reflect the original information of mineral evolution (temperature, pressure, pH, etc). The external factors, such as temperature and pressure in the process of evolutiondirectly affect the quantity and size of inclusions, which are related to the properties of crystals. In this paper, terahertz time-domain spectroscopy was used to detect KCl crystals with different growth temperature, and the curves of time domain, frequency domain and absorption coefficient were obtained. In addition, microscope was used to observe the inclusions inside samples. The results showed that there was a corresponding relationship between the absorption coefficient and the area of the inclusions. The larger the area of inclusions was, the more coefficient the THz absorption was, and the analysis of the influence of the area of inclusions on terahertz optical parameters provided a new and effective way for the study of fluid inclusions. The innovation of this paper is to use THz-TDS to quickly detect the change of the area of inclusions in crystals. The results indicates that terahertz time domain spectroscopy, as a new nondestructive testing method, has a unique advantage in the detection of inclusions.