Abstract: The development of a multi-physics coupling program for the fast reactor fuel assembly was presented in this paper. The fuel performance analysis code KMC-Fueltra and the sub-channel analysis code KMC-Subtra were coupled based on the ICoCo interface to obtain more accurate thermo-mechanical boundary of fuel rods in the fuel performance analysis. KMC-Fueltra is a steady-state and transient performance analysis code developed for the fuel element in the liquid metal cooled fast reactor, aimed at predicting and evaluating the evolution of thermal, mechanical, and irradiation performance of the fuel element during service. KMC-Subtra is a sub-channel analysis code developed based on the thermal-hydraulic characteristics of lead-cooled fast reactors, used to calculate key parameters such as temperature and pressure of the coolant. ICoCo is a universal coupling interface that defines the essential functions required in a numerical simulation program. This allows the main code to arbitrarily call these functions when coupling with other codes, enabling operations such as time step advancement, mesh mapping, and data transfer. Numerical simulation programs must provide these functions to the ICoCo interface according to the evolution of the physical problem and specify the appropriate method to be called at specific times. The Supervisor code written in C++ language was used as the main code to run the coupling program. KMC-Fueltra and KMC-Subtra were wrapped to different essential functions in the form of dynamic library or static library according to the ICoCo interface, which could be called by the Supervisor code. The operator separation scheme was used for time coupling, and the region overlap scheme was used for spatial coupling. The half-mesh method and full-mesh method, which could be chosen by the specific research problem and computer hardware condition, for parallel calculation of multiple fuel rods were established using OpenMPI library. The mesh mapping and data interpolation were implemented based on the MEDCoupling library, and the maximum value principle was used as the interpolation principle. The outer surface temperature of the cladding, the coolant temperature, and the coolant pressure were chosen as the coupling parameters between KMC-Fueltra and KMC-Subtra based on the physical equations. Verification of the data transfer interface of the coupling program was conducted with these three key parameters from the ORNL-19 fuel assembly benchmark case. The outer surface temperature of the cladding is completely consistent, the relative error of the coolant temperature is 0.15%, and the relative error of the coolant pressure is 1.4%, which can prove the accuracy of the coupling program. This study can provide reference and guidance for the multi-physics coupled service performance analysis of fuel rods in the liquid metal cooled fast reactor.