Transient electromagnetic fields can couple to overhead power transmission and distribution lines, causing damage to the connected equipment. In real cases, incident electromagnetic fields exhibit different spatial and temporal waveform distributions along the line. Normally, to ensure the accuracy of the simulation results, a large number of observation points are needed along the line, which is difficult to obtain. This paper proposes a new approach to address this problem. The proposed approach is based on Agrawal et al.’s coupling model, in which the amplitude and phase distributions of the horizontal exciting electric field along the line are, respectively, represented by the Fourier series and polynomial functions in the frequency domain using two kinds of least squares methods. As a result, the source terms along the line can be accurately evaluated starting from a limited number of observation points. The proposed approach has been validated experimentally.