Abstract: The excitation function measured from radio interference test corona cages should be calibrated before being applied to radio interference calculation of the transmission line. The calibration factor is usually obtained by calibration experiments. However, the accuracy of calibration experiments is affected by the test condition and environment. This paper analyzed the theoretical calibration factor based on the circuit analysis method, which is more economic and convenient than calibration experiments. The distributed parameter model of the corona cage test circuit was established, and according to the independence of the corona discharge pulse in space and time, curves of the calibration coefficient for the corona discharge source at different position were obtained. Thus, the calibration coefficient was calculated, which was compared with the one obtained from proportion of capacitance. The results showe that the theoretical analysis method presented can be applied to calibrate the excitation function from corona cage tests, and it is economical and practical.