Abstract: The performance of sensing optical fiber plays a key role in maintaining the stability of a fiber optical current sensor（FOCS）. In order to overcome the impact of the random birefringent, a special spun high birefringence elliptical polarization-maintaining fiber was fabricated and applied to design a FOCS sensing coil. The structure and working mechanism of the sensing fiber were described, and the ability of anti-interference was analyzed. At a rated current of 1200 A, over a temperature range from 40℃ to 70℃ the sensor was tested and achieved a high accuracy. The amplitude measuring error was lower than 0.05% and the phase measuring error was within ±2’ respectively; when the frequency of current measured was between 0Hz and 1500 Hz, the amplitude measuring error was within 0.2%. During eight hours, of vibration at 40m/s~2, the amplitude measuring error was always kept within ±0.05%, and the phase measuring error was always kept within 2’. The FOCS achieved by the special optical fiber is put into operation in the 800 k V Ling Zhou Convertor Station.