Radio interference, as an important part of the corona effect of conductors, is one of the important indicators for environmental assessment of transmission lines. In this paper, based on the Excitation Function Method, a three-dimensional radio interference distribution calculation model for a transmission line that takes into account the sag of the conductor is established. This model divides a finite-length conductor into several micro-segments, and calculates the radio interference generated by each micro-segment. The radio interference field strength at the measured point can be obtained by summing the line integrals, and finally the three-dimensional distribution of radio interference in the transmission corridor is obtained. Using this model, the radio interference field distribution characteristics of an actual transmission line are calculated, and the radio interference field strength value at a distance of 20m from the side phase is compared with the measured value under heavy rain conditions at a long-term observation station. The error is only 1.38%, which illustrates the accuracy of the radio interference calculation method proposed in this paper. Finally, the influence of the conductor sag on the distribution of radio interference is analyzed. It is shown that the change of the conductor sag mainly affects the amplitude of radio interference in the power transmission corridor without changing the position where the amplitude appears. And the further the vertical distance from conductor, the smaller the impact.