Excitation saturation of core is deepened when the transformer is subjected to DC invasion, which causes excitation current to distort and harmonic components are increasing significantly. In this paper, the characteristics of excitation harmonic and core saturation are studied, and a criterion for core saturation instability of the transformer DC disturbance is proposed. Based on the principle of electromagnetic coupling, the state equation of transformer in AC / DC hybrid mode is constructed, then the identification method of excitation current is studied based on port electrical information. The harmonic of excitation current is analyzed and its variation law is summarized, furthermore, the discriminating method of core saturation instability is studied by analyzing the saturation mechanism of the transformer under DC disturbance. The electromagnetic characteristics of transformer are analyzed through calculation and dynamic simulation experiment and the effectiveness of this method is verified, which provides a feasible method for discriminating the saturation instability of core under DC disturbance. This work is supported by National Natural Science Foundation of China (No. 51507027).