In this paper, after investigating the current research status of lightning protection design of transmission lines, we proposed an improved electrical geometry modeling method for the calculation of trip out rate considering the lightning shielding effect between lines in a dense transmission lines. On this basis, we made the validation of the results considering the feasibility of multiple line calculations. Through this work, we verified the validity of the proposed computational approach. And based on that method, we calculated and analyzed the change rule of lightning shielding effect between two transmission lines. The distance between the transmission lines, the height of the transmission lines from the ground and the slope of the ground on both sides for the transmission lines have a great influence on the calculation results.