In order to analyze the transient inrush current characteristics of the transformer, it is necessary to obtain the accurate leakage inductance value on each side of the transformer. In this paper, the transformer loop equation is first deduced, the proportional relationship between the primary zero-mode current and the delta circulating current is got, and the self-leakage expression is obtained. For the problem that the delta winding current of 220 kV transformer cannot be measured, the primary unsaturated phase current is used instead of the circulating current based on the balance of the delta winding circulating current and the primary unsaturated phase current. After determining the quasi-proportional coefficient of the selected unsaturated phase current and the zero-mode current, the unsaturated time interval is further determined, and the average value in the interval is used as the obtained proportional coefficient. Finally, the self-leakage is calculated by the above expression. The improved transformer model simulation and on-site recording verify the method. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of Guangdong Power Grid Co., Ltd. (No. GDKJXM20162461).