Abstract: Based on the security region theory, this paper proposes a relatively complete security early warning process for regional integrated energy system (RIES). First, this paper constructs the RIES security region model and introduces the RIES security boundary calculation method, including the AC security boundary and DC security boundary. Then, fuzzy inference and fuzzy comprehensive evaluation methods are introduced to assess the severity of violation in early warning. Next, the security early warning method is proposed, which comprehensively considers the N−0 and N−1 security to send the early warning or alarm signals. The method proposed includes the selection and assessment of early warning indices, the level classification of security early warning, the analysis of early warning, and the prediction of security trend. Finally, the effectiveness of the proposed method is tested on case studies. The security early warning method proposed in this paper has important application value for improving the security management and control ability of RIES.