As massive distributed energy resources (DERs) are integrated into distribution networks (DNs) and the distribution automation facilities are widely deployed, the DNs are evolving to active distribution networks (ADNs). This paper introduces the architecture and main function modules of an integrated distribution management system (IDMS) and its applications in China. This system consists of three subsystems, including the real-time operation and control system (OCS), outage management system (OMS), and operator training simulator (OTS). The OCS has a hierarchical architecture with three levels, including the local controller for DER clusters, the optimization of DNs incorporated with multi-clusters, and the coordination operation of integrated transmission & distribution (T&D) networks. The OMS is developed based on the geographical information system (GIS) and coordinated with OCS. While in the OTS, both the ADN and its host transmission network (TN) are simulated to make the simulation results more credible. The main functions of the three subsystems and their interaction data flows are described and some typical application scenarios are also presented.