In order to meet the urgent needs of online metrology of the ultra-high current in the field of industry, national defense and key scientific research, a value transfer method of the ultra-high current standard based on the range self-expansion characteristic of the fiber-optic current sensor (FOCS) was presented. The mathematical model of the sensing coil was established by utilizing the differential Jones matrix methods. The effect mechanism of the linear birefringence on the range self-expansion characteristic of the sensor was analyzed, and the excellent range self-expansion capability of the FOCS with a spun high- birefringence sensing fiber coil was proved. By means of the magnification effect of the number of fiber loops to the Faraday effect, the sensor can be calibrated with the relatively low equivalent ampere-turn current. The flexible interferometer-type FOCS utilizing the digital closed-loop signal-detecting scheme was developed. The spun high birefringence optical fiber was packaged into a sensing element, which can be bent into a sensing coil without opening the current-carrying bus bar. It is shown by the experimental results that the sensor achieves the accuracy within ±0.1% for the DC equivalent current between 10 kA and 210 kA. For a power-frequency equivalent current of 25 kA, the scale factor error of the sensor can meet the need of 0.2S accuracy class specified in IEC 60044-8. The bandwidth of the sensor is more than 10 kHz.