Abstract: The statistical model is used as a typical method for vacuum insulator stack flashover probability calculation. We have calculated the stack flashover probability and the g factors by which a flashover will change the voltages on the each of the insulator rings with different conditions. The calculated results show that the real performance of insulator stack characteristics with the multi-stage and large circumferential transit time. The matrix a in the statistical model was predigested to reduce the g calculation time and make the probability calculation accurate. Finally, analysis based on fixed distance between insulation rings showed that there were some optimized ring numbers with maximum voltage amplitude and electrical field amplitude. This conclusion could be used for evaluation of insulation ring number in insulator stack design.