Abstract: In this paper, a broadband window sensor is developed to measure the very fast transient over voltages (VFTO) that caused by the operation of the disconnecting switch in 1100 kV GIS model of UHV experimental base. Based on the theory of capacitive voltage divider, flat\|plate films capacity is used on low voltage side, and distributed capacity of GIS conducting rod and low\|voltage electrode is applied to high voltage side. The experiment is carried out by using of 1100kV UHV GIS. Experimental results show that the rise time of VFTO is 10ns, and the main vibration frequency is 563kHz and 1.37MHz, on which the high\|frequency oscillation with 7.8MHz and 13.2MHz is superimposed.