Terahertz spectrum is corresponding with vibration and rotation of liquid molecules. It is suitable to identify and research the liquid molecular dynamics. As a powerful spectral detection technology, Terahertz Time-Domain Spectroscopy(THz-TDS) is widely applied in solution detection. The absorption coefficient, refractive index and dielectric function of solutions can be extracted based on terahertz time-domain spectroscopy. Water and NaCl exist in most biological tissues, which are very important for life. In this paper, water and NaCl solutions are detected under room temperature by THz-TDS technique in the range of 0.2-1.5 THz. The terahertz dielectric function of the water and NaCl (sodium chloride) solutions is fitted by Debye model, from which the dielectric relaxation time can be obtained. By means of dielectric relaxation process, it is found that the characteristic time of molecular movement and the information related to the liquid molecular structure and movement are obtained. The terahertz dielectric function of the water and NaCl solutions is fitted by Debye model, where the dielectric relaxation time of water and NaCl solutions is 9.6 ps and 10.7 ps respectively. By means of dielectric relaxation process, it is found that the characteristic time of molecular movement and the information related to the liquid molecular structure and movement are obtained.