The operation data obtained abroad indicates that shielding failure in UHV transmission lines mostly accounts for the tripping-out accidents introduced by lightning striking the transmission line. Based on the discharge theories of long air gap and randomness theory, a leader progression model of lightning shielding failure is presented in this paper. The random characteristics of the downward and upward leader are simulated in this model. The ground slope angel is also considered in this model by using coordinate transformation. Moreover, the system voltage is also taken into consideration in this model. The simulation results show that the good agreement between this model and the field data. And the results suggest that return striking exist obviously in UHV transmission line.