Very fast transient overvoltage (VFTO) is a special electromagnetic transient phenomenon when the isolation switch is switched in a gas insulated substation (GIS). VFTO affects the normal behavior of secondary equipments through conduction and radiation. In order to explore the interference characteristics of VFTO on secondary equipments,a self-made interference measuring device is used in this paper to conduct field measurement of the common mode interference on the secondary side of the potential transformer (PT) in 1 000 kV GIS. Secondly,the performances of five time-frequency analysis methods are compared,and the time-frequency analysis of the measured waveforms is carried out using the synchronous compression wavelet transform with the better performance. The measured results in this paper show that the peak-to-peak voltage of common-mode interference voltage on the secondary side of PT reaches up to 9.65 kV. The time-frequency analysis results show that the 7.8 MHz frequency component has the highest amplitude,so it is the dominant frequency component of PT secondary side interference throughout the waveform. The results in this paper provide references for the electromagnetic immunity test and electromagnetic protection design of secondary equipment in GIS.