Dynamic characteristics are essential to investigate and predict the responses of electric transmission line tower (TLT) system under dynamic actions, e.g., fluctuating winds. To efficiently solve the dynamic characteristics of the line tower system subjected to winds, a modal-synthesis-based modeling method is developed based on the free interface theory. Taking a system composed of two-pan conductors and three support towers as the example, its natural frequencies and modal shapes under average winds are computed by means of the line-tower component division, the interface compatibility between the tower and line, and the component modal synthesis (CMS). Comparisons between the free-interface CMS method and the finite element method (FEM) demonstrate that the CMS method significantly enhances computational efficiency, with results showing high accuracy and consistency.