To ensure the stability and reliability of fiber optical current transformer (FOCT) long-term operation, this paper proposes a method for real-time monitoring of key state parameters of core components without affecting the normal operation of FOCT, such as second harmonic amplitude, fourth harmonic amplitude, power of super luminescent diode (SLD) light source, return signal power, and modulation signal. This method uses a 2×2 coupler with a splitting ratio of 95∶5 to split the optical path of FOCT. One end of the coupler monitors the light emitted by SLD, and the other end monitors the return signal of FOCT. And through the combination of actual measurement and theory, the feasibility and accuracy of the proposed method for FOCT state monitoring are verified. This method can comprehensively diagnose and troubleshoot equipment faults based on changes in FOCT key state parameters.