Abstract: In recent years, the development of electric vehicles has shown a trend of rapid growth，and the emergence of large-scale new distributed energy generation technology,profound changes have been brought to the analysis,design and control of the traditional distribution system,the nonlinear behavior of distribution network becomes more complicated,aggravated the pollution of the power system.Therefore, developing comprehensive analysis tools to evaluate the power supply capability of active distribution network, and then eliminating negative effects through coordinated control, it has important research value to build a comprehensive evaluation model of power supply capability. In this paper ,application of the unascertained measure theory,considering the uncertainty size of the new energy generation forecast and the uncertainty of the typical flexible load response behavior based on the charging of a scale electric vehicle,comprehensive and reasonable evaluation of power supply capacity. The integration of all kinds of distributed power sources and the charging facilities of electric vehicles bring great challenges to the operation of the distribution network,distributed generation of renewable energy in particular,because they are generally connected to the distribution network locally in suitable climates, it is easy to cause overload, voltage limit and voltage stability of distribution lines and transformers in local power grid.So it is reasonable to discuss the three dimensions of the voltage limit, the limit of the thermal limit and the breakdown of the voltage.The concept of the total transfer capability is proposed.the power system, under the premise of security constraints, specifies the maximum power transmission capacity on the path of the transmission circuit,It is the basis for determining the available transfer capability (ATC),ATC refers to the remaining, commercially available transmission capacity in the actual physical transmission network on the basis of the existing transmission contract.The calculation of ATC is mainly deterministic and probabilistic. In the active distribution network, the power system operation constraint (AGC) ensures the balance of power generation and load, maintains the power system frequency quality and the physical constraints of the active distribution network.The mathematical model of the power supply capacity from the new energy generation node to the load node is studied and established.