An ultra high voltage auto-transformer has been analyzed in this paper. The maximum voltage of this auto-transformer is 750 kilo voltage, and the capacity is 500 millions volt-ampere. This paper primary introduced the calculation and analysis for the main insulating structure of single-phase power auto-transformer. The whole electric field of transformer be calculated by the finite element analysis software ANSYS. The average electric field intensity is calculated by the self-program with APDL language of ANSYS.