In order to study the reason why the waveform attenuation degree is quite different from the actual measurement in the VFTO calculation of UHV GIS substation, this paper first introduces the generation mode and waveform characteristics of the measured VFTO waveform of a UHV substation in China. Then the possible attenuation factors of the VFTO waveform, such as the arc resistors, basin insulators, GIS elbows and GIS bushings are analyzed. The original IEC recommended model is improved based on possible electromagnetic leakage or radiation loss. Finally, the overall simulation model of GIS substation is established. Through the equivalent conversion of the measured waveform, the simulation conditions are unified with the measured waveform, and the simulated waveform characteristics under different models are obtained. The results show that after considering the waveform attenuation factors studied in this paper, the attenuation degree of the simulated waveform is more consistent with the actual measurement, and the rationality of the proposed model is verified. This work is supported by Science and Technology Plan of SGCC ‘Study of Accurate Simulation of Overvoltage in UHV Project and Study of a New High Frequency Overvoltage Suppression Wire (No. GY71-16-006)’.