Abstract: To study thermal characteristics of optical units in optical fiber composite cables and thermal losses caused by fiber macro bending, a two-dimensional coupled model of multiple physical fields based on solid heat transfer, solid mechanics, and wave optics was established using the finite element simulation software COMSOL. Thermal losses of two types of G652 and G657 optical fibers with auxiliary groove structure used in cables were compared under normal and macro bending conditions. Results showed that thermal field distribution and stress distribution of the two types of optical fibers were completely the same at the same ambient temperature, but thermal losses of G652 fiber was greater than G657 fiber in both two states, which indicated that different structures and refractive index distributions had a significant impact on thermal losses of optical fibers, and structure and refractive index distribution of G657 fiber with auxiliary groove structure were more reasonable and suitable for indoor use.