To avoid the error of CT polarity inversion since the transmission lines have been put into operation, an on-line discrimination method during the transient process of transmission line’s no-load closing is proposed in this paper. No-load closing model for the single-phase transmission line is established firstly, and the mathematical relationship between electrical quantities and line parameters during closing process is described by differential equations. Line parameters such as the inductance or resistance are regarded as unknown quantities, and the characteristics of their values are analyzed when solving the differential equations under different CT polarities. Then the calculation values are normalized,so that the discriminant method is applicable to different transmission lines. In order to quantify the distribution characteristics of normalized equivalent values, forward and backward coefficients are defined. According to the ratio of these two coefficients, CT’s polarity in single-phase system can be discriminated. On this basis, combined with the mathematical relationship among the three-phase currents and the characteristics of Karenlaucer phase-mode transformation matrixes in different forms, the polarities of CTs in three-phase system can be identified. Finally, the effectiveness of the method is verified by simulation.