A new supply capability model for distribution systems is proposed. Firstly， based on N -1 safety criterion， definitions are given to the related indices including the distribution system maximum supply capability， substation supply capability， network transfer capability， available supply capability， etc. Then the maximum supply capability model is developed and divided into two parts， the maximum supply capability sub-model and the load balance sub-model. After the problem is transformed into one of linear programming， the maximum supply capability is obtained by step by step solution and the balanced distribution of the main transformer loads is attained. Finally， a case study is given to verify the effectiveness of the proposed model and improves the analytic calculation accuracy of the maximum supply capability.