An improved leader progression model (LPM) for analyzing the shielding failure performance of overhead transmission lines is established, and implemented in calculating the shielding failure rate and shielding failure flashover rate of 500-kV transmission lines. This paper discusses the inadequate parts of the existing LPM and presents improvements to meliorate the model for applications on extremely high voltage (EHV) transmission lines. The model is improved mainly in three aspects: 1) the background electric field produced by the lightning cloud is simulated by multielectrode configuration, 2) different inception criteria of upward leaders for shielding wires and phase conductors are chosen, and 3) the impact of power frequency operating voltage on the shielding failure flashover rate is considered. The improved model is applied to analyze the shielding failure performance of 500-kV transmission lines in China considering the flat and hilly terrain conditions. The shielding failure flashover rates by existing models and the improved model are calculated and compared with the operating data of 500-kV transmission lines. It is shown that the predicted result of the improved LPM is in accordance with the operating experience. The improved model is suitable for the lightning shielding assessment and applicable in the design of EHVAC transmission lines.