Abstract: Through research in very fast transient overvoltage（VFTO） test circuit of UHV AC test base, it is found that the decay time of calculated VFTO single breakdown waveform is more than that of measured waveform. This result will affect the evaluation of hemi-wave time of VFTO standard waveform through simulation. So the research of loss mechanism is needed. Based on the poor considering of radiation loss of bushing conductor in CIGRE recommended model and other literatures, this paper firstly analyzed the traveling wave mode of current distribution on bushing conductor, deduced the calculation formula of radiation resistance and average characteristic impedance of bushing conductor and build transmission model of bushing considering radiation loss. Then the frequency domain calculation method of VFTO was proposed in order to handle the frequency dependent parameter of radiation resistance. The calculation results in VFTO test circuit show that the radiation resistance of bushing conductor is much bigger than the arc resistance and the bus-bar resistance at main frequencies, and meanwhile the decay time is much more close to that of the measured waveform. Therefore the rationality of the proposed calculation method has been verified.