Abstract: Based on Martin equation, the paper summarizes the calculation method of flashover probability of vacuum insulator stack, which has many insulator stages and large azimuthal transit time, under a given voltage waveform. 1/4 power time dependence and 1/10 power area dependence are chosen in analysis. Then the flashover probabilities of vacuum insulator stack of SATURN and Z machine are calculated as examples. The theoretical criteria for similar vacuum insulator stack design are offered. The paper also detailedly discusses the uncertain factors and existent questions, and establishes the base for further optimizing of the analysis methods.