Abstract: Large-scale security and stability control system (SSCS) is characterized with wide-area distribution, multi-layer control, numerous components, and complicated function realization. The conventional reliability assessment indicators, such as malfunction rate, rejection rate, and failure rate, are too general to accurately measure the reliability level of SSCS. Based on the constitution of SSCS, the local abnormal forms are classified. In view of engineering application, the control efficacy reduction modes of SSCS are also classified, and the reliability indicators are further defined based on the control efficacy reduction modes. To assess the reliability of SSCS, the expected abnormal sets are firstly established, and then their induced control efficacy reduction modes and corresponding probability of occurrence are analyzed. The practical application indicates that the proposed reliability indicators, clear in definition, simple in calculation method, and flexible in assessment content and scope, can be used to study the effects of such factors as the architecture, the configuration and constitution of devices, the operation modes, and the operation logic and mechanism of core functions, on the SSCS reliability. The results of this research can provide an important basis for improving SSCS reliability.