Due to the limitation of linear analysis of suspended pendulum control system with small swing angle, this paper proposed the method of analyzing the structural dynamic response considering the nonlinear property of pendulum, in which the linear stiffness of pendulum is revised to be nonlinear. Taking a 500 KV transmission tower as a numerical example and comparing the results at linear and nonlinear conditions, it can be found that the swing angle of pendulum may exceed the limitation of linear assumption and the nonlinear analysis can simulate the structural dynamic system with large swing angles of pendulum perfectly. The parameters of control system including excitation periods, pendulum length and mass ratio were studied on the influence of the seismic reduction ratio of transmission tower. Three seismic records were used to calculate the response of transmission tower with suspended pendulum. The results showed that the mitigation effect of suspended pendulum on structural response is very obvious under different seismic excitations.