Due to lack of the "Source-network-Ioad" global perception and multiple time-scale situation awareness, the current distribution network cannot meet the requirements of the active distribution network (ADN) such as active control and active management. An ADN situation awareness system based on distributed monitoring and multi-source information fusion is proposed in this paper. By constructing a multi-level distributed monitoring system based on Internet of Things (lOT) technology, monitoring objects of distribution network are expanded to achieve global awareness in spatial scale. In the light of the randomness and volatility risk of distribution network operation, it is necessary to enhance observability of situation awareness in time scale. Thus, the situation awareness technology of distribution network based on multi-source information fusion is provided, which is constituted of the multi-source information fusion, source-load forecasting, fast simulation analysis, risk assessment and warning and visualization modules. Finally, the operation cockpit visualization technology enables operators to efficiently capture accurate distribution network security situation trend. The ADN Situation Awareness System Demonstration Project is currently being carried out by State Grid Jiangsu Power Company, the actual operation indicates that it can effectively enhance the operation situation awareness and risk early-warning capability.