Based on the mathematical discreteness of the heat state equation, a high precision FOG(fiber-optic gyroscope) thermal model was built as system angel. The building course of the system thermal model of the fiber-optic FOG was introduced, including of building structure geometry model, inner heat source model and fiber coil model which is sensitive to the heat. The method building model is not only applicable to this type of FOG, but also to other type whose structure and composition is analogous. The heat point distribution and temperature field distribution of the system was studied by steady state thermal simulation. The inner temperature change during the startup at 25.6��, -40�� and +60�� was analyzed by instantaneous state simulation, and the validity of the model was approved by the experiments. At last, some significative conclusions were gained. And this work will advance the batch production of the high precision optical fiber gyroscope.