Abstract: The design and analysis method of PTS vacuum insulator stack are presented. The voltage waveforms of each-level are obtained by full circuit simulation. The shapes of the components (grading rings, insulator rings, anode and cathode conductors) in each level and the design of the water and MITLs interfaces, were optimized by the electrostatic analysis code. The 3-D and 2-D voltage variation of each insulator of level-A is compared, and the difference is neglectable. The time-dependent performance of insulator stack was evaluated using 2-D PIC code. The voltage variation change caused by electron emission on vacuum side of grading rings is serious, and can greatly affect the whole stack flashover probability.