Lead–bismuth fast reactors are considered promising for industrial demonstration of fourth-generation nuclear energy systems due to their significant advantages in nuclear fuel breeding and transmutation, so the code development is urgent. Based on the COSINE software package, this study first modifies fluid properties, pressure drop models, turbulent mixing coefficients, and heat transfer correlations to adapt the program for the thermal–hydraulic analysis of sub-channels in liquid metal-cooled reactors, while ensuring that the software’s ability to compute results for other working fluids remains unaffected, thereby preserving the integrity of its original functionality. Subsequently, the modified program is preliminarily validated by comparing it with relevant experimental data and CFD numerical simulation results. By comparing with the KYLIN-II lead–bismuth loop rod bundle experimental results, it is observed that the program provides an accurate prediction of cladding temperature, with the computational error within 10 K, which preliminarily validates the accuracy of the COSINE software package in calculating coolant temperature and cladding outer surface temperature. Additionally, by comparing with Fluent numerical simulation results, it is found that the average outlet temperatures predicted by both codes are very close, indicating that the COSINE software package results are somewhat conservative, and preliminarily validating the reasonableness of the turbulent mixing model in the COSINE software. This provides support and reference for thermal–hydraulic calculations of fuel assembly design parameters.