Abstract: This paper proposes an electromagnetic transient model of the two-core Sen transformer (TCST) based on the unified magnetic equivalent circuit (UMEC) in order to analyze its electromagnetic characteristics. Firstly, the TCST may be divided into the two parts of the series winding and the exciting winding so that their magnetic equivalent circuits are established based on their iron core structures respectively, according to the magnetic coupling relationships among the internal windings of the TCST. Secondly, with the mathematical expressions of the electromagnetic transient models derived by the UMEC method for the two parts separately, an electromagnetic transient model of the TCST can be acquired by the combination of the series winding and the exciting winding in the light of their electrical connections in the TCST. Finally, with the help of the MATLAB and the PSCAD/EMTDC, the analytical calculation and the time-domain simulation have been carried out, proving the effectiveness of the proposed model. In comparisons with the results involving the currents, the voltages and the power flows in the existing published papers, it is found that the current flowing through the tap-changers in the TCST is about 50% of the line current, which can effectively reduce the manufacturing cost of the tap-changer. Meanwhile, the actual adjustment step length of the TCST is about 0.5 times smaller than that of the single-core Sen transformer (SCST), which may lead to more accurate performance in the TCST.