The ratio of the pore volume in the drug tablet to the total volume of the tablet in the natural state is called porosity. In the production process of drug tablets, due to the physical and chemical properties of raw materials, human factors and environmental factors, the formation of pores is inevitable. Porosity is an important characteristic of drug tablets. The porosity size will affect the disintegration, dissolution and bioavailability of tablets. At present, the common methods for measuring the porosity of tablets, such as mercury intrusion method, helium specific gravity method, infrared spectroscopy, etc., can not achieve nondestructive and rapid detection of the porosity of tablets. For this reason, this paper proposes a method to detect the porosity of a single drug tablet by using continuous terahertz waves. Two standard planar drug tablets are used as research objects respectively. The signal transmitted through each tablet is measured in the frequency range of 500~750 GHz using vector network analyzer, and the packaging phase value of each tablet is extracted from the measured S parameters. Then phase unwrapping and correction are carried out to obtain the true phase value of the tablet, and the effective refractive index of the tablet is obtained by calculating the phase difference between the tablet and the air. At the same time, the theoretical model of zero porosity approximation (ZPA) is used to link tablets effective refractive index and porosity. The relative errors between the calculated porosity of the two tablets measured by the vector network analyzer and the standard porosity measured by the gas displacement method are 7.3% and 5.3% respectively. The experimental results show that measuring the tablets porosity using continuous terahertz wave is feasible. The THz wave method for measuring the porosity of tablets is simple, practical, non-destructive and fast, which lays a foundation for rapid, sensitive and non-destructive porosity measurement in the future pharmaceutical tablet manufacturing and production.