The existing IEEE (Institute of Electrical and Electronics Engineers) standards, which are developed based on antenna theory, can only explain the reradiation interference mechanism of transmission lines within a limited frequency band of 0.5~1.7 MHz. To study the reradiation interference at higher frequencies, the generalized resonance theory was introduced in this paper, and a method for predicting resonant frequency based on the energy balance of electromagnetic fields was proposed. By taking the power transmission lines, tower arrays and antennas in a large-scale space as a generalized closed system, we were able to derive the expression of the generalized resonance factor of a open electromagnetic system based on the complex Poynting theorem, and hence to obtain the resonant frequencies of reradiation interference, which were the zero points of the generalized resonance factors. Unlike the conventional methods, which take towers as half-wave antennas, this method is able to predict the resonant frequencies of reradiation interference in mid-wave frequency band. The method was used to calculate the examples in IEEE standards. The results show that at frequencies below 1.7 MHz, the error between the prediction results and the resonant frequencies measured using scaled models is no more than ±0.169 MHz.