Abstract: An active distribution system (ADS) contains a lot of distribution generations and flexible loads. Compared to the traditional distribution network, ADS has greatly increased the supply capacity and supply flexibility, which will bring new challenge to the operation security of power distribution system. In this paper, a supply capability model for active distribution systems based on N-1 security criterion is built considering impacts of factors, such as distributed generation, distributed energy storage system and electric vehicles, on the power supply ability and flexibility of the distribution network. In addition, a two-level fuzzy comprehensive assessment method is proposed based on multi-sets of indicators, and a multi-level situation judgment index system of security posture forewarning is built by combing such factors as load characteristics, power supply characteristics and climatic, to judge security posture forewarning grade of active distribution system. Taking solving case by linear planning software as example, the validity of model and methods are verified.