We develop a mathematical model of a fiber optic current sensor and deduce a mathematic equation of measurement accuracy of the sensor. The influence of the loop gain variation on the measurement accuracy is theoretically investigated. Further, we develop a method for online monitoring of the loop gain. The loop gain value is demodulated and transmitted to the monitor in real-time as one of fault monitoring points for system monitoring. A stable loop gain controlling technique is developed to automatically adjust the loop gain, thereby eliminating the accuracy degradation caused by the loop gain change in the long-time system running. Results show that loop gain value monitored online as well as the scale factor error and phase error change as the preamplifier gain changes, consistent with the simulation results. Furthermore, the loop gain value monitored online remains stable, and appropriate measurement accuracy can be achieved while utilizing the proposed stable controlling technique. These observations denote the effectiveness of the proposed online monitoring and stable controlling technique for loop gain.