The invention discloses a method for assessing the lightning flashover risks of regional power grid lines, which comprises the following steps that: through calculating the lightning-resistant level of each base rod tower and combining with a probability distribution function of lightning current amplitudes of a region, the lightning trip-out rate of each rod tower is obtained, and the average lightning trip-out rate of a line is calculated; then, through combining with an automatic reclosing failure rate and a failure rate of manual forced energization, a permanent failure rate of the line is obtained; and through considering the weight coefficient of the line in a power grid structure, the permanent failure load loss coefficient of the line at the specific time of a power grid and in an specific operation mode of the power grid, and the risk occurrence probability of other influencing factors, a lightning flashover risk assessment model of a regional power transmission grid is established. According to the invention, the shortcoming that the absolute numerical value of the lightning trip-out rate can not really reflect the lightning disaster risk level of each line is overcome, and the lightning flashover risk level of each rod tower of lines is assessed through comprehensively considering the differences among the lightning motions, topographical features, line structures, insulation configuration, lightning protection measures and the like of each line corridor in the power grid, so that more comprehensive and thorough assessment results are obtained.