Abstract: Due to limitations of the standard method, electrical geometry model, improved electrical geometry model and leader progress model in calculating the shielding failure trip-out rate, it adopts an improved calculation method, this method is based on the catenary equation and the formulas of shielding failure trip-out rate located at different positions of the mountain, the altitude and ground inclination of transmission corridor at any point along the span direction are taken full account, the actual height among the ground wire, the conductor wire and the ground and the angle of shade of transmission lines at any point are obtained, then uses MATLAB to develop applications to calculate the shielding failure trip-out rate of transmission lines at any point in any span with comparing the actual lighting data. Simulation and Calculation results verify the correctness of the improved calculation methods in this study. It provides a reliable basis for lightning protection design and renovation of overhead transmission lines under the high-altitude mountainous complex terrain conditions.