Switching operations of a disconnecting switch (DS) or a circuit breaker (CB) in a gas insulated substation (GIS) can stimulate outside GIS strong radiated electromagnetic fields. In order to assess the status of GIS through its characteristic parameters, a method of extracting the characteristic frequency-band energy of the electric field (E-field) based on wavelet packet analysis is proposed. Using the bior5.5 wavelet basis function, the radiated E-field signals of CB in good condition and those of CB in which insulation fault happens are separately decomposed into 4 layers, and the normalized energies of different frequency bands are extracted; then it’s found that the frequency band below 31.25MHz is the characteristic band of radiated E-field. By comparing the energies in the characteristic frequency bands after a random CB operation with that of the typical radiated E-field obtained when GIS totally is in good condition, insulation faults of CB can be detected and the operating state of GIS could be assessed.