We investigate mode-coupling effects of terahertz whispering-gallery modes (WGMs) in a multi-mode resonator using an extended transfer matrix method. Coupling effects between two WGMs are successfully observed on a well-designed high-Q terahertz Teflon ring resonator that supports low-order WGMs. The extended transmission matrix method is adopted to model, analyze, and reproduce terahertz single-band resonance and asymmetric dual-band resonances caused by mode coupling. Moreover, the terahertz splitting effect based on coupling of two modes in a multi-mode resonator is theoretically illustrated. This detailed analysis not only contributes to understanding physical phenomena, such as mode splitting, but also helps in identifying parameters when designing terahertz devices, such as dual-band filters.