Lightning shielding failure is the main reason causing lightning flashover of Jiangcheng EHVDC transmission line because of the designed ground wire protection angle. Based on the electric-geometry model, a simulation model including the coupling ground wire is established by using MATLAB. The effect of the coupling ground wire at different height and horizontal position on lightning shielding failure flashover rate is analysed. Results show that coupling ground wire can reduce the lightning shielding failure flashover rate and the lightning shielding performance increases with the increase of the height of the wire and the horizontal distance between the axis of the tower and the coupling ground wire.