Abstract: The ring beam is the main supporting structure of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) reflector. The flexible cable net structure bearing reflector units are connected with a ring beam structure by 150 ear-plate joints. During construction and observation, we tried to ensure the stiffness of the main support structure by the base of the theory of structural reliability and the requirement of reflector accuracy. Since the temperature load plays a controlling role for such a large-scale structure, we used the finite element simulation software and tested reflector precision to find the rules of structure stiffness, by releasing temperature stress, adjusting the stiffness of the structure and reserving deformation space, and then made the parameters to be a real time parabolic so as to adjust the deformation observation.