110kV　minimized　gas　insulated　substation　(GIS)　has　been　widely　used　in　China,　due　to　its　great　electrical　properties.　The　closing　or　opening　operation　of　disconnector　switches　(DS)　and　circuit　breaker　(CB),　and　single-phase　ground　fault　are　all　probable　to　cause　the　very　fast　transient　overvoltage　(VFTO),　which　is　a　threat　to　the　power　system.　Compared　to　the　structure　of　higher　voltage　CIS,　three-phase　electrical　equipment　in　110kV　minimized　GIS　is　placed　in　the　same　box.　So　we　must　establish　the　three-phase　model　of　GIS　and　then　calculate　its　VFTO.　This　paper　referred　to　110　kV　minimized　GIS　power　system.　Using　the　Electro-magnetic　Transient　Program　(EMTP),　numerical　simulation　and　analysis　of　VFTO　in　different　DS,　CB　operation　modes　and　single-phase　grounding　fault　were　carried　out.　And　the　data　of　VETO　under　different　operation　modes,　including　the　frequency,　amplitude,　steepness　and　waveform　were　obtained.　Then　it　calculated　and　analyzed　the　influence　on　VFTO　of　length　of　branch　bus　and　transformer　entrance　capacitance,　in　order　to　provide　reference　basis　to　the　structure　design　of　GIS　and　analysis　of　VFTO.