To measure the transition section and outlet current of LTD single branch verification device, a B-dot current probe is designed and calibrated. The radial transmission line is used to calibrate the current probe offline, and the corresponding verification experiments are carried out to verify the effectiveness of the calibration. The experimental results show that the sensitivity deviation of the probe is about 1% due to the errors of probe installation depth and angle. When the distance between the cathode and the anode is greater than the diameter of the probe hole, the B-dot calibration result is independent of the distance. For the coaxial transmission line with large diameter involved in the device, the simulation and calibration results of radial transmission line are effective. According to the experimental results of the LTD single channel verification device, the difference of anode current from the four transition sections to the outlet is very small, while the cathode current decreases gradually.