Abstract: Operational data show that the fault probability of fiber optic current transformer(FOCT) is significantly high in extreme environments (temperature is –45~85 ℃, vibration acceleration is 15 m/s² or more), so it is necessary to study the impact of extreme environments on the performance of FOCT. On the basis of analyzing the working principle of FOCT, we focus on discussing the structural characteristics of the core module of FOCT and the influence of the extreme environment, and establish the FOCT transmission model. According to the real working environment of FOCT, the influence of extreme environment on its measurement accuracy is analyzed. The results show that the increase of temperature, the increase of fiber length, and the larger vibration acceleration will facilitate the FOCT ratio difference to increase and the measurement accuracy to decrease. Especially in the extreme environment, the measurement error is large and cannot meet the requirement of 0.2S. In order to verify the reliability of the model, temperature and vibration influence tests are carried out. Aiming at the lack of assessment of the extreme environment in the existing test, we propose the temperature test method with increased measurement points and the vibration test method with increased vibration response test and vibration endurance test. Comparison of the test results with the simulation results shows that the two results are consistent, and the trend of deviation current waveform change is relatively consistent. The research of the paper provides a useful reference for the reliability of FOCT.