The nonlinear vibration equations of twospans transmission line coupled with suspension insulator string were derived, where the geometric nonlinearity of transmission line was considered. Solutions under different working conditions were obtained by numerical analysis. The displacementtime curves illustrated the characteristics of nonlinear coupled vibration and internal resonance of the system. In the case of small transmission angle, the value of the angle had no obvious effect on the amplitude of transmission line, but the static equilibrium position of system was offset.