In order to evaluate characteristics of VFTO and its harm to the main electrical equipment, we have done a great quantity of emulational calculations integrating with a certain 750kV GIS Substation in Northwest Power Grid of China. In the process, preliminarily, we have mastered the distribution of VFTO on main parts inside and outside of GIS and made a conclusion that the VFTO amplitude limit is in withstand capability of the relative equipment of this substation. We performed measurement on site based on the calculations, mastered important characteristic parameters, such as actual amplitude, wave shape, frequency and so on of VFTO in 750kV GIS substation. The measured results on site have instructive significance for the switching operation of disconnector of 750kV GIS, meanwhile, it made us accumulate valuable experience and data for actual measurement of VFTO and switching operation of UHV 1 100kV GIS.