In this paper, a method used for determining the number of equivalent pi sections of the transmission line model according to the frequency range of interest and the model accuracy required is proposed. By analysing the frequency characteristics as well as the characteristic frequencies of different models, factors influencing the discrepancies between the distributed parameter and the lumped parameter models are studied. Time-domain simulation confirms the effectiveness of this method, and general conclusions concerning the pi section lengths of the line models used in different transient studies are drawn.