The all-fiber optical current transformer (FOCT) has become a key current measurement equipment in power systems due to its non-saturating magnetic performance and excellent electromagnetic compatibility. However, its measurement performance is significantly challenged under extremely cold conditions, mainly because the material and physical properties of sensing fibers change at low temperature. This study establishes a geometric model of circularity-preserving sensing fiber loop composed of core, cladding, and stress rods, and constructs a multi-physical field coupling model integrating temperature and optical fields. Through detailed simulations, the temperature distribution of sensing fiber in a − 45 °C low-temperature environment is analyzed, along with the variation law of refractive index and loss characteristics. The results reveal that the fiber demonstrates excellent thermal conductivity, ensuring uniform temperature distribution even in extreme cold. The thermal expansion discrepancy between the core and cladding induces internal stress, leading to an increase in refractive index and loss. This research provides a theoretical foundation for optimizing the design of sensing fiber loop, which is crucial for enhancing the measurement accuracy and reliability of FOCT in extremely cold environments.