Abstract: Thermal-hydraulic transient analysis code for reactor system is an important tool for thermal-hydraulic calculation and safety evaluation. To develop the thermalhydraulic analysis code for the heliumxenon (HeXe) cooled space reactor, the HeXe physical properties were expanded into RELAP5/MOD4.0 and the suitable heat transfer correlation for HeXe was also added. The comparison of calculations by the modified code with the experimental data was conducted. The results show that the default Sutherlands law will introduce large error for the HeXe physical properties calculation. Predictions of Nusselt number by DittusBolter formula overestimate the experimental data for HeXe flow and heat transfer, resulting in an unsafe numerical margin for the wall temperature. Calculations of both pressure drop and heat transfer coefficient by the expanded code agree well with the experimental data, verifying the correctness of code development and the feasibility for calculating HeXe flow and heat transfer. This research can lay the foundation for the system-level code development.