Abstract: The flashover probability of vacuum insulator stack (VIS) is calculated based on statistical model. The calculation results show that the probability reduces with lower peak voltage of VIS, shorter effective pulse duration, smaller material constant and smaller stack radius in a certain range of VIS voltage. The flashover probability is also calculated with the consideration of magnetic flashover inhibition (MFI) under different critical ratios. The critical ratio is defined as the ratio of electric field strength to magnetic induction strength when MFI begins to be effective, and is changed according to different angles between insulator and electrode considering the relation of the three-phase point electric field strength of cathode and the average electric field strength of VIS. The si