With the rapid development of the power system and continuous improvements in the power-related industries, the operational reliability advantages of GIS substations are gradually being recognized and applied. However, the compact structure of GIS substations can lead to the generation of fast transient overvoltage during switch operations, which can potentially cause severe electromagnetic interference to nearby secondary equipment and the GIS itself. In recent years, experts in related industries have been researching this issue and attempting to mitigate it using various measures. Therefore, it is essential to accurately model the source of fast transient overvoltage, study its time-domain and frequency-domain characteristics, and provide a reference basis for electromagnetic compatibility design of power equipment. Taking a 550kV gas-insulated substation as an example, an equivalent circuit model of the substation was established, and the waveform of fast transient overvoltage generated during circuit breaker operations was studied. The time-frequency characteristics of interference signal waveforms were analyzed using synchronous wavelet transform.