To meet the needs of emerging applications such as online software testing, power grid state visualization, and personnel training, a co-simulation scheme for ADPSS and the D5000 system is proposed, establishing a large-scale power grid simulation platform that closely resembles a real power system environment for security operation and intelligent dispatching. First, based on actual large-scale power grid data, an automated modeling method and the ADPSS-CIM/E model conversion tool suitable for the co-simulation system are developed. Second, a virtual remote terminal unit (RTU) simulation device is designed and developed to generate composite RTU models, along with protocol conversion to emulate RTU communication. Third, external interaction methods during the ADPSS simulation process are studied, and a "four-remote" (telemetering, teleindication, telecommand, teleadjusting) response module is developed within the core computational program. Then, data interface programs are developed to ensure simulation efficiency and data compatibility across heterogeneous systems. Through parallel execution of multiple computation and interface processes, real-time regulation of a power grid with tens thousands of nodes and integrated dispatching/control is achieved. Finally, based on the co-simulation platform, a 12 000-node online power grid model is constructed, and tests on "four-remote" interaction simulation and state estimation are conducted. The results validate the effectiveness of the closed-loop co-simulation of ADPSS and the D5000 system.