In order to investigate the mechanical response and interpret the key collapsing reason of old transmission towers under unfavorable conditions of strong winds, an automatic numerical simulation method has been developed with emphasis on the 110 kV ZS-type of overhead transmission towers. The static bearing capacity of components under different load conditions as well as dynamic characters of the structure were analyzed in this study based on the proposed simulation method and mode superposition theory. The results show that the middle section of the ZS-type transmission tower is the weakest section. Thus, the relatively weak middle section combined with insufficient anti-twist capacity of the tower is the main cause of its structural collapse. On this basis, structural reinforcement and performance improvement can be carried out, which were verified by the numerical model.