High-voltage electric transmission tower may collapse under the strong earthquake and studies on the collapse mechanism, routine and capacity of transmission tower-line system are important for the structural design of tower. In this paper, a progressive collapse analytical procedure for the system is proposed based on the finite element method (FEM). During this procedure, the mass of the elements is still retained rather than removal after elements lose the load-bearing capacity. The proposed procedure is coded using the user subroutine VUMAT and then implemented in the advanced finite element program ABAQUS. A three-dimensional finite element model for the system of three towers and four-span lines is created. By using the coded subroutine, the collapse analysis of the tower-line system under the strong earthquake is performed. Collapse processes along longitudinal and lateral direction are studied, respectively. Furthermore, the influences of ultimate strain and strain rate effect of materials on the collapse mode and capacity are studied. The results indicate that the collapse analysis of the tower-line system by using the proposed procedure can provide collapse mode and vulnerable points for use in seismic performance and retrofit evaluation of structure. It is found from the numerical modeling that the influences of ground motion and ultimate strain on the collapse modes are apparent. The collapse-resistant capacity of system increases remarkably with the increase of ultimate strain and influences of strain rate on collapse routine and capacity are tiny in analytical results.