The load current is connected to the meter for measurement via the current transformer (CT), if the CT secondary circuit is artificially changed to achieve the purpose of stealing, impedance characteristics of CT secondary circuit will change. Through the CT secondary circuit in series test CT, the high frequency detection signal of the specific frequency band generated by the STM32 microcontroller is injected from one end of the test CT secondary side, and the high frequency signal returned from the other end of the test CT secondary side is sampled and processed by the STM32 microcontroller. If CT secondary circuit status is different, the signals collected by the microcontrollers are also different, the corresponding amplitude-frequency characteristics are also different. Experiments show that the device can quickly and accurately detect the normal, open, short circuit and abnormal state of the secondary circuit of CT with large difference in inductance of the secondary coil.