The effect of optical phase modulation instability on the scale factor (SF) in Sagnac interferometer-type optic-fiber current sensor (OCS) is theoretically analyzed in this letter. The inherent temperature dependence of the optical phase modulation and the drive signal noise are the main reasons of increasing SF error. By means of employing calibration process and feedback configuration in modulation depth of the optical phase modulator, the SF error is improved within the range of 1%, verifying the class 0.5 S, which successfully inhibits the effects of those disturbances.