Abstract: A 500 kV GIS VFTO test platform was established and validated for functionality. A complete VFTO measurement system was built, with waveforms captured by oscilloscopes and data saved automatically. VFTO tests were completed under no-load, extended busbar, simulated residual voltage, and severe cold conditions. The waveform characteristics and amplitude probability distribution of VFTO were analyzed. Using ATP-EMTP software, simulation models were created and numerical calculations performed for VFTO under different conditions, with results closely matching measured data. The study shows that VFTO amplitudes are mainly in the range of(1.6～2.4)p.u., and are related to factors such as pre-charged DC voltage level, busbar length, and chamber pressure.