Abstract: Electric geometry model, as a kind of engineering model solving lightning shielding problems, has been widely used in assessing shielding failure lightning performance of HV and EHV transmission lines. However, with the increase of UHV line voltage level and the growth of the air gap, the results calculated by the previous EGM is not consistent with the observed data. Hence, this paper corrects the equation between the lightning current and the striking distance, based on the data of negative polarity 50% discharge voltage from the rod-rod gap-20/2500μs switching impulse test of which the distance of air gap ranges from 1-10 m, and the return stroke velocity distribution obtained by Idone was newly applied. In addition, According to the character that, when the gap distance exceeds 4m, the size relationship of negative polarity 50% discharge voltage between the rod-rod gap and rod-plane gap changes, the striking distance factor of ground has been corrected based on the test data. With these improvements, an improved EGM applied to large-sized transmission line has been proposed, which is applied to calculate the lightning stroke rate of 500 k V and 1000 k V transmission line in Japan and the lightning stroke rate of 1000 k V transmission line in China. It was found that the results calculated by the improved EGM are closer to the actual observations in Japan, especially the lightning stroke proportion of each phase conductor.Thus,this improved EGM is more suited for large-sized transmission lines and can provide a more reliable foundation for lightning shielding design.