Abstract: The phenomenon of fluid temperature fluctuation is common in fast reactor. The center measurement column of fast reactor is located at a position that will undergo severe fluid temperature fluctuation, which will reduce the life of reactor materials and threaten the safety of reactor. In order to study the effect of fluid temperature fluctuation on the life of center measurement column lower head, the center measurement column model was simplified as a flat model and a set of thermal fatigue assessment method was proposed. The fatigue damage factor of the lower head was calculated by Miner’s rule. The results of fatigue damage factor at lower head meet requirements of the Miner’s rule and can provide a reference for material selection and thickness selection in engineering applications.