An optical-structural-thermal model was built to analyze the thermal stability of an optical-structural system in the Fizeau interferometer. The analyzing process of the model, random distribution of environment temperatures, and the optical performance of the system were researched. Firstly,a random model for environment temperatures was built based on the thermal control system, and the thermal distribution and deformation of the optical-structural system were computed. Then, the surface forms and curvature radii of the optical component were fitted with the Zernike polynomial,and the fitted optical system was analyzed with the optical software. Finally, a random model to indicate the relationship of optical performance and temperature was built and the thermal stability of the main frame in the interferometer was discussed through analyzing the random model. Results indicate that the repeatability of the optical-structural system is 0.016%λ under believable level of 2σ when the temperature is (22±0.1)℃, which satisfies the requirements of the Fizeau interferometer for stability.