Abstract: After investigating the research status of transmission line lightning protection designs, this paper proposes an improved electrical geometric model to calculate the shielding failure tripping rate considering the shielding effects between lines in dense transmission channels and verifies its feasibility, mainly including the optimization of the calculation idea and the modification of the ground dip angle processing method. Based on the proposed calculation method, the calculation and rule analyses are carried out for three transmission lines under the parallelled working conditions. It is found that the adjacent lines play a dominant role in the shielding effect between lines, and the strength of shielding effect does not change linearly with the line spacing, but change with a rapidly changing range. On this basis, the calculation of the line spaces and the heights to the ground considering the actual line trends and sags is proposed. The proposed method is used to calculate in the actual project, which proves the feasibility of the method. This method has a certain guiding role in the lightning protection reconstruction of the transmission lines.