With the widespread application of gas insulated substation（GIS）

due to its numerous merits

such as large miniaturizing effects

good anti-contamination characteristics

easy maintenance and good environmental adaptability.Harm of the very fast transient voltages（VFTO） caused during disconnector switches（DS） or circuit breaker（CB） operation becomes more obvious with the increasing level of GIS voltages.This paper refers to 750 kV GIS of Guan Ting in northwest China power system.Using the Electro-magnetic Transient Program（EMTP）

numerical simulation and analysis of VFTO in different operation modes GIS are carried out

and the data of VFTO under different operation modes

including the frequency

amplitude

steepness and waveform of VFTO are obtained.According to the calculation data

insulation coordination and safety margin of GIS and other devices are analyzed

and theoretical data and the key measuring region for the coming field practical measurement are provided.