With the wide use of Extra High Voltage Gas Insulated Substation (GIS), Very Fast Transient Over-voltage (VFTO) generated by switching operations threatens the insulation of some equipments even the security of the whole power system. The transformer is the main casualty of VFTO. For a 800kV GIS, the model has been built up and the parameters have been selected in this paper. VFTO magnitudes as well as the frequency spectrum of different running modes have been calculated using Electro-Magnetic Transient Program(EMTP). The protective effects of shunt resistor of the switches, Metal Oxide Arrester (MOA) and R-C surge suppresser have been studied. The results indicate that the shunt resistors could accelerate the attenuation of VFTO, so VFTO magnitudes and gradients reduce greatly. MOA could only reduce the magnitudes. R-C surge suppresser not only could limit VFTO magnitudes, but also could reduce the gradient of VFTO.