Gas insulated substation (GIS) is widely used in urban and densely populated areas. Moreover, GIS equipment is used in all UHV AC transmission projects. The operation of the isolating switch in GIS produces very fast transient overvoltage (VFTO). When the frequency component contained in the VFTO is matched with the resonant frequency of the winding type device, a resonant overvoltage of high amplitude is generated between the turns of the winding type device, which affects the insulation safety of the device, so it is necessary to perform spectrum analysis on the VFTO. VFTO is a kind of non-stationary signal, Fourier transform cannot meet the requirements of its spectrum analysis, while time-frequency analysis (TFA) methods are suitable for the analysis of non-stationary signals. In this paper, the spectral characteristics of VFTO are analyzed by continuous wavelet transform (CWT). By comparing the TFA results of CWT, short-time Fourier transform, Wigner-Will distribution and smooth pseudo Wigner-Will distribution, the good TFA performance of CWT is verified.