Based on electromagnetic theory and related research, a fitting formula for calculating the resonant frequency of the evanescent mode of a T-cavity terahertz parallel-plate waveguide is proposed. The resonant frequency of the evanescent mode under different parameters is studied using the fitting formula and COMSOL software. Experimental results show that the results obtained by the two methods are in good agreement, and the relative deviation does not exceed 0.3%. The fitting formula provides a fast and simple calculation method that can accurately obtain the resonance frequency of the evanescent mode.