A discrete dynamic model of fiber optical current transformer is established. The relationship between the scale factor error, phase error of the transformer and the system loop gain is theoretically investigated. It′s determined that the loop gain drift is one of the main reasons that lead to the degradation of the long-time operation accuracy of the transformer. A method for online loop gain monitoring is presented which can monitor the change of loop gain in real time. The monitoring value can be used as one of the fault monitoring points for long-time operation of the transformer. Based on it, the model correctness is verified by experiments. Results show that on the condition of ensuring the static test accuracy, the greater the system loop gain value is, the smaller the influence of loop gain fluctuation on scale factor error and phase error of the transformer will be. When the system loop gain is 0.05, the loop gain fluctuation should not exceed 10% to ensure that the transformer measurement accuracy in long-time operation is less than the required error limit.