Abstract: Compared with traditional thermal neutron reactors, fast reactors have higher fuel enrichment and harder energy spectrum, and the Doppler broadening effect dominated by U-238 during reactor operation is weaker, so the influence of core structure deformation on reactivity accounts for a larger share of the overall reactivity feedback. Therefore, an accurate assessment of the reactive feedback effect of thermal expansion of the reactor core is an important part of the safety analysis of fast reactors. For pulse reactors, the critical calculation of the model before and after thermal expansion was carried out based on the Monte Carlo method, and the thermal-mechanical coupling calculation was carried out based on the finite element analysis method, so as to carry out the nuclear thermal coupling calculation and analysis of the thermal expansion reactivity feedback effect of Godiva-I pulse reactor and Godiva-IV pulse reactor. The calculation results show that the reactivity temperature coefficient of fuel expansion are in good agreement with the experimental values, and the maximum relative error is within 5%, which verifies the applicability and accuracy of the coupling method.