The invention relates to a step voltage calculation method under a symmetrical double-core phase shift transformer load condition. The calculation method comprises a step of determining rated current IN and series transformer power supply side voltage Us according to the rated pass capacitance S and the rated voltage UN of a phase-shifting transformer, a step of determining series connection transformer load side voltage UL according to the rated current IN,overall leakage impedance Z and power factor angle phi, and combined with the phase shift angle in an empty load, determining series connection transformer power supply side current Is and load side current IL, a step of determining excitation transformer primary side current IE1 and second side current IE2 according to the series connection transformer power supply side current Is and the load side current IL, a step of determining excitation transformer primary side voltage UE1 according to the series transformer power supply side voltage Us, the load side voltage UL excitation transformer primary side current IE1, and secondary side current IE2, and thus determining excitation transformer secondary side voltage UE2, and a step of determining a load tap-changer level voltage Ustep according to the excitation transformer secondary side voltage UE2. The invention provides a method for calculating overall leakage impedance Zn and a phase shift angle an in an empty load in a selected n gear of the phase shift transformer.