A mathematical model was built on the principle of the three-phase to four-phase impedance matching balance transformer and the theory of multi-wiring transformer. Meanwhile, the performance characteristic of the transformer under different loads was analyzed. The transformer applied in AT traction power supply system wiring scheme was presented,and its electrical characteristic of wiring scheme was discussed. The research has shown that the impedance matching transformer can not only realize the balanced symmetrical conversion of three-phase to four-phase, but also has an obvious superiority in restraining the negative sequence current caused by the lopsided load.