Abstract: The unsteady temperature field of fuel element was analyzed. Considering reactor neutron kinetics and heat transfer inside fuel element as well as the physical properties coupling temperature, the theoretical model was proposed. Using the steady difference method, the temperature variation and distribution in the cylindrical fuel element were obtained under the conditions of step reactivity and temperature feedback. The results are useful and significant for the thermal-hydraulic design and safety analysis of the nuclear reactor, especially for nuclear-powered reactors.