Abstract: To explain the generation mechanism of the severe imbalance of three-phase inrush current of high-impedance transformers, the concept of core “saturation degree” is defined. This can reflect the imbalance of three-phase inrush current. With the help of phasor theory, the analytical method for imbalance of three-phase inrush current is proposed based on a saturation degree dynamic phasor diagram. Based on this method, the variation law of the saturation degree of each phase iron core with the opening and closing angle and time can be intuitively obtained, and the boundary angle of the extreme imbalance can be quickly identified. The numerical simulation results at different opening and closing angles show that the trend of imbalance degree and zero-sequence current magnitude is basically the same. The simulation and wave record validate the consistency of the boundary angle of extreme imbalance and peak value of zero-sequence current. Based on the parameter characteristics and boundary angle of a high-impedance transformer, the phenomenon of serious imbalance of three-phase inrush current in high-impedance transformer is explained by the increase of zero-mode inrush current on the primary side and the decrease of circulating current of secondary side.