The correct wiring of a line protection current transformer (CT) secondary sampling circuit is a precondition for correct protection operation. There is difficulty in polarity checking during the operation and start-up of the line. Thus the boundary limit condition of the polarity checking using the steady-state current of the line under limited load and no-load is first analyzed. There is also difficulty of no-load polarity checking when the condition is not met. For this a CT polarity checking method for line protection based on no-load charging transient current is proposed. The CT relative polarity of three phases is preliminarily checked by the local transient current with large amplitude. Then, the transient current of the opposite terminal is calculated based on the measured value of the local terminal and the Bergeron line model to determine the phase with the wrong polarity of the local terminal. When the CT polarity is correct, the calculated current of the opposite terminal is theoretically zero. If not, it will have a large amplitude. The problem that the traditional method cannot take effect when the load is insufficient is solved by the proposed method. The effectiveness of the method is verified by simulation and wave recording. This has been incorporated into the engineering application scheme.