With the increasing penetration of renewable energy resources (RES) and flexible loads (FL), their potential in enhancing the security regulation of MV/LV distribution systems has become increasingly evident. Considering the impacts of extreme renewable energy output conditions and flexible load behaviors, this paper proposes a distributed smart grid security region (DSG-SR) model that incorporates RES and FL. First, the operational characteristics of RES and FL are analyzed, and their corresponding mathematical equations are established, followed by discussions on constraints related to normal operation and security. Next, treating the operational modes of RES and FL as state variables, a DSG-SR model is established considering the extreme output scenarios of RESs and the FL response cost constraints. Finally, the application of the DSG-SR model is illustrated through two-dimensional cross-sectional plots, revealing the mechanism by which RES and FL influence the DSG-SR. The proposed model can serve as a practical tool for security analysis in distributed smart grids, providing key technical support for secure and stable operation and for the design of market mechanisms in distribution systems with high shares of RES and FL.