Dielectric measurements of rock plug samples have been widely employed as an important basis for the in-situ evaluation of reservoir geological parameters. Key information, including saturated porosity, can be reflected by dielectric dispersion characteristics. In order to accurately characterize the dielectric dispersion behavior of plug samples and improve the inversion accuracy of in-situ data such as dielectric logging, a dielectric testing system based on microwave resonance theory was developed. The operating frequency range was set to 100 MHz~1 GHz to match that of dielectric dispersion logging tools. High measurement accuracy was achieved through the application of microwave resonance principles and calibration with standard materials. The experimental results show that the dielectric properties of dry rock samples are nearly non-dispersive, whereas the dielectric response is mainly affected by pore-water content and mineralization. A generally linear correlation is observed between permittivity and saturated porosity. Both the dispersion of the measured data and the influence of pore-water salinity are found to increase with decreasing frequency. In addition, the dielectric dispersion characteristics of plug samples are also found to be affected by differences in geological composition and microstructure. To further analyze the underlying mechanism, model fitting based on effective medium theory (EMT) was carried out. The results indicate that dielectric dispersion below 500 MHz is mainly controlled by electrical conductivity and is closely related to the clay content and cation exchange capacity (CEC) of the plug samples.