In this article, we used FDTD optical simulation software to simulate a terahertz whispering galley mode resonant cavity with a circular ring structure. We employed fiber coupling method and set the diameter of the fiber to 200 um. Simulate a resonant cavity with a radius of 5 mm to 10mm in the 0.325–0.5 THz band. The ring width is set to 0.5mm, and the wall thickness of the 5mm radius circular resonant cavity is controlled to be 0.5 mm, 0.4 mm, 0.3 mm, 0.2 mm, and 0.1 mm, respectively. The resonant cavity uses quartz material and adopts 2D simulation to save simulation time. We observed the spectral lines and electric field intensity distribution of the terahertz whispering galley mode resonant cavity, and characterized five index parameters after the resonance phenomenon occurred: resonant wavelength, free spectral range, full width at half maximum, quality factor, and mode volume. Finally, the resonance characteristics of resonant cavities of different sizes in the terahertz frequency band were discussed. The results indicate that terahertz resonators exhibit good performance in specific frequency bands. This provides a theoretical basis for designing high-performance terahertz WGM resonators and demonstrates their potential application value in the field of precision sensors.