Abstract: The mitigation system for anticipated transient without scram (ATWS) is one of the essential systems which assure the safety of China Advanced Research Reactor (CARR). Once the anticipated transient occurs while the reactor scram can not be realized, the safety of the reactor can be fulfilled by ATWS mitigation system which is functioned to shutdown the reactor. The function of ATWS mitigation system relies on its high reliability which is rendered close attention in this paper. The research on the reliability of ATWS mitigation system was carried out, using the reliability analysis methods including failure modes and effects analysis(FMEA) and fault tree analysis (FTA). And the relevant model was established. The qualitative and quantitative analyses of ATWS mitigation system were performed. The fault probability and minimal cut sets of malfunction were obtained. And the weak links were found. Improvement measures and suggestions were presented. The reliability has already reached the engineering design requirement of CARR. The design of the ATWS mitigation system was verified. The work in this paper can lay the foundation for the analysis and verification of other systems of CARR.