The sodium-cooled fast reactor has obvious advantages in improving the utilization rate of uranium resources and minimizing the high level radioactive waste. There is a strong feedback mechanism in the neutronics and thermal of the reactor. In this study, the Monte Carlo codenal RMC and subchannel analysis program VERSA are used to carry out the neutronics-thermal coupling calculation. Taking the fast reactor as the research object. In order to improve the efficiency and accuracy of nuclear thermal coupling iterative calculation during the information transmission process, a full core physical thermal–hydraulic nuclear thermal coupling interface program was developed, realizing the automatic recognition, extraction, mapping, and conversion of power distribution and temperature field information in the core physical and thermal–hydraulic calculation results. At the same time, the multi-box component parallel calculation function was implemented in the full core thermal–hydraulic calculation, and the full core thermal–hydraulic calculation can be completed in 2 min, improving the calculation efficiency. The three-dimensional power and temperature field distribution in the entire core can be intuitively displayed through the post-processing of the calculation results. Through the research on the three-dimensional fine-grained nuclear thermal coupling calculation of the entire core, the comprehensive and accurate power distribution and temperature distribution of the core were obtained, providing a basis for further optimizing the core flow allocation and improving economic efficiency. It also provides a basis for accident prevention and safety evaluation.