In view of the insufficiencies of present lightning shielding theory (electrogeometric model and leader propagation model), the physical process of lightning stroke and the scatter of shielding failure on transmission line were studied. Some new knowledge of lightning shielding of transmission line and the simulation theory is proposed. A lightning stroke simulation model of transmission line, which is based on the probability of discharge, is proposed. The model was applied to calculate the rate of shielding failure of 500 kV ZM1-39 transmission line. Calculated results using the proposed model show good agreement with field experiences