By studying the thermal-induced phase shift mechanism of an interferometric fiber-optic gyroscope (IFOG) sensing coil, a novel generalized expression based on a three-dimensional (3D) model is proposed. Compared with the traditional pure Shupe effect model, the simulation results show that the new 3D model, including elastic strain and the elasto-optical effect, can describe the thermal effect of the coils more accurately. Experiments with temperature change rates between 40°C and 70°C are performed to verify the effectiveness of the proposed generalized expression. The results of our work can guide researchers in identifying countermeasures to reduce the thermal-induced bias error in IFOG.