The abnormal state of the secondary circuit of metering current transformer will not only cause economic losses to power grid enterprises, but also lead to safety accidents. In order to realize the accurate on-line monitoring of the secondary circuit state, through the theoretical study of the circuit equivalent circuit under different states, a secondary circuit state monitoring method based on the 10kHz impedance characteristics is proposed. The circuit 10kHz impedance measurement technology based on micro transformer coupling and vector voltage current method is adopted, and the current transformer circuit state monitoring device is developed. The experimental results show that the developed device can accurately identify the normal connection of secondary circuit, short circuit of secondary terminal, secondary open circuit and primary bypass under the full working conditions of current transformer transformation ratio of 30/5~2000/5, ambient temperature of (-40~60)℃ and secondary current of (0~120%)I2n.