Abstract: The influence of two short equivalent lines length on the accuracy of the asymptotic method for analysing electromagnetic field coupling to transmission line at high frequency is investigated in detail. Firstly, the induced current distribution along the transmission line in terms of different lengths of two short lines is analyzed through numbers of calculations. It is indicated that the calculated induced current along the line by asymptotic method is not reliable when any one of the short equivalent lines is half wavelength long. Secondly, the dependence of the two short equivalent lines on the whole transmission line length is studied. Results show that when the length of the transmission line is much longer than the distance between two conductors, it is necessary to increase the suggested lengths of the equivalent short lines in order to get more accurate induced current along the line.