Abstract: When the power grids with close voltage levels, the design of tie auto-transformer would become difficult due to the low effectiveness coefficient. This paper proposes a novel design of auto-transformer with common core for constant flux and variable flux. Through three-dimensional electromagnetic field simulation and calculation, the no-load main flux distribution, voltages of windings as well as important parameters such as winding short-circuit currents and short-circuit impedance of the transformer are analyzed calculated. The simulation results basically coincide with the results from estimating, which illustrates the feasibility of the new structure. By adopting this transformer with new structure, the primary material cost and no-load loss as well as transport weight can be effectively reduced. Furthermore, with simplified structure, the transformer can be manufactured with lower difficulty level, and the operation reliability would be enhanced consequently.