Overhead transmission lines are the significant component of electric power system,where the over current can be coupled with the transient electromagnetic field or excitations (such as high-altitude electromagnetic pulse). As the coupling path of strong electro-magnetic interference,overhead transmission lines cause serious interference to the power system. Among the existing modelling methods,the classical transmission line theory may generate large error when dealing with the high-frequency coupling problem,where the cross dimension of the transmission line is not electrically small. Numerical full-wave method (such as moment of method) which relies on the grid subdivision with low efficiency when dealing with long transmission lines. Moreover,the number of cables is usually large in electric power system,and the ground are considered as the lossy ground. Therefore,to address the above mentioned problems,an asymptotic method is proposed to calculate the high-frequency coupling current along overhead transmission lines in electric power system. Based on the asymptotic theory with high calculation efficiency,the scattering and reflection process are introduced to quantify higher-order model components. In addition,the arbitrary number of wires,arbitrary parameters of the ground and different excitations are considered to derivate the current expression. Finally,the validity and reliability of the proposed method are tested using the full-wave simulation and antenna irradiation experiment. The proposed method can quickly calculate high-frequency coupling current,which can provide theoretical basis and data support for protection and electromagnetic effect study of overhead transmission lines.