This paper presents the VFTO calculation and analyzing results of XiMeng UHV pilot project in China. The result shows that the maximum voltage amplitude of VFTO in GIS is 1981kV, so that the pre-insert switching on off resistance is not necessary to installed on disconnector. In this case , it will reduce the probability of GIS failure operation and the potential supply current, and the safety and reliability of UHV GIS system will be improved. Meanwhile,the VFTO of a transformer in GIS substation is not high when the transformer is connected to GIS by overhead line.