Aiming to the problems of traditional active electromagnetic transformer such as easy magnetic saturation, poor stability and anti-interference ability, and limited installation, etc., this paper designs the optical fiber current transformer to measure current by rotation angle based on Faraday magneto-optic effect; HB Spun optical fiber is used as sensing element without saturation and can be used for high current measurement. The designed transformer uses the optical reciprocity loop to eliminate the interference of temperature and optical fiber defect on the measurement of optical rotation angle, and uses reflector to enlarge the optical rotation angle to four times, which can realize accurate measurement of small current; Sensing element uses flexible fiber ring with shape variability characteristic, which helps for measurement of current in complex space. The paper compares flexible fiber ring with different loops to standard current transformer, the results show that optical reciprocity loop can eliminate the interference of temperature on the current measurement and the accuracy of all-fiber current transformer is 0.5 in the range of -5 ℃～70 ℃, which can realize the accurate measurement of small current.