Abstract: Supercritical CO₂-cooled reactor, as a promising reactor, is at the conceptual design stage. The physical and mathematical models of the main parts of the primary circuit of supercritical CO₂-cooled micro-modular reactor (MMR) designed by Korea Advanced Institute of Science and Technology (KAIST) were established, and the transient and safety analysis code TRA_SCR for supercritical CO₂-cooled reactor was preliminarily developed with FORTRAN language. Steady state analysis shows the stability and dependability of TRA_SCR code for KAIST MMR. The variations of main parameters for loss of flow and reactivity insertion accidents without protection were calculated, and the transient safety characteristics of this system were preliminarily studied. The results show that KAIST MMR has strong inherent negative feedback characteristics, and under these two accidents, the temperatures of the cladding, fuel and coolant do not exceed the safety limitation. However, the outlet temperature of the core coolant is close to the safety limitation in loss of flow accident without protection, which shows that the outlet core temperature is the key factor limiting the safety performance of the system.