Abstract: In order to realize the power supply of small nuclear reactors in remote areas, the sub-channel analysis program BRESA of supercritical carbon dioxide (S-CO₂) fuel assembly based on Modelica language was developed in this paper. According to the fuel assembly model of 37 rods, the axial and radial physical and thermal parameters of S-CO₂ fuel assembly were calculated, and the flow mixing effect between channels was introduced to obtain the distribution law of thermal hydraulics parameters in the axial and radial directions, The enthalpy rise and temperature rise changes of various typical channels were studied. The results show that the enthalpy rise and temperature rise changes of the channel in the middle of the fuel assembly are high, while the enthalpy rise and temperature rise changes of the channel at the edge of the fuel assembly are low. Through comparison with relevant literature, it is proved that the thermal hydraulic parameters of this study meet the design requirements and the correctness of the program is verified. On the basis of steady-state calculations, the transient conditions of flow rate changes were analyzed, and the response characteristics of various parameters of fuel assemblies under transient conditions of linear increase, linear decrease, and sudden change in flow rate were analyzed. The response characteristics of reactor power response and control system were investigated and analyzed, and the spatial variation trends of enthalpy and pressure with time were obtained. The results indicate that under transient conditions, the reactor power will vary with flow fluctuations, which is related to the negative feedback effect of the control system and temperature. The pressure and enthalpy values also fluctuate and change when the flow rate changes, which is influenced by the dual effects of flow rate and power changes. Under transient conditions of sudden flow changes, enthalpy and pressure undergo sudden and significant fluctuations, with the maximum fluctuation of outlet enthalpy reaching 12%, posing a challenge to the safe operation of the reactor. The research results of this paper provide theoretical support and technical guidance for the design and research of supercritical carbon dioxide reactor systems.