Due to the magnetic hysteresis characteristics of transformer cores, residual flux often remains in the core after the transformer interruption. After various electrical experiments are conducted, the flux may become more severe, which can lead to serious inrush currents when the transformer is re-energized. The peak value can reach 6–8 times the steady-state current of the transformer. Among existing methods for inrush current suppression, phase-selective closing strategies are effective and easy to implement, but requiring accurate measurement of the transformer's residual flux, especially after various testing procedures. To address this, this paper investigates the magnetic hysteresis characteristics of transformer. Based on an improved Jiles-Atherton (J-A) hysteresis model for transformers, the characteristics of the hysteresis loop are analyzed. By examining the different states of the hysteresis loop under different voltage excitations, a residual flux measurement strategy for transformer based on low-frequency voltage excitation is explored. This strategy provides an effective means of measuring residual flux for inrush current suppression through phase-selective closing of transformers.