Supercritical CO₂ (S-CO2) Brayton cycle is recently receiving significant attention as a next generation power conversion system for wide range of energy applications due to its high efficiency with a mild temperature heat source (around 450~650°C), compact footprint and so on. The main reason why the S-CO2 Brayton cycle has those advantages is that the compressor operates near the critical point of CO2 (30.98°C, 7.38MPa) to reduce the compression work. Therefore, the S-CO2 Brayton cycle is generally designed to set the compressor inlet condition close the critical point for making the best use of the behavior of CO2 like an incompressible fluid near the critical point. However, this operating condition can cause the compressor inlet condition to be two-phase during transient situations such as a pre-cooler accident, seasonal change of ultimate heat sink, CO2 leakage accident, etc. This may degrade the system due to decrease in efficiency and turbomachinery damage. In spite of the active development for the S-CO2 Brayton cycle, research on the cycle transient behavior, especially in case of CO2 compressor inlet condition variation, is still in its early stage. In this study, the research for the S-CO2 system operation analysis is conducted under the conditions of the cooling heat transfer capability variation. The gas system transient analysis code GAMMA and the S-CO2 compressing test facility called SCO2PE (Supercritical CO2 Pressurizing Experiment) are utilized for this study.