Abstract: The buckle of one or more bracings in a transmission tower leads to stress redistribution and overloads in the adjacent bracings, causing the potential collapse of a transmission tower and the normal functioning of an entire power line. Therefore, the large economic loss is inevitable in such an event, and thus it is important to properly assess transmission tower bracings. In this paper, we used the parametric resonance theory to determine the dynamic instability regions and the excitation coefficients of transmission tower bracings. The results show that the dynamic instability regions become smaller and the minimum excitation coefficients of the main and diagonal bracings become larger with the increase of mean wind. Then, the relationship between the minimum excitation parameters and the dynamic instability regions was used to assess the stability of bracings; moreover, wind tunnel test data were used to validate the analytical model. The results show that the diagonal bracings with significant deformations in local vibration modes are much more likely to undergo parametric resonance and lose bearing capacity than the main bracings.