Abstract: The traditional way of calculating power supply capability of distribution system,based on the interconnection is formed by main transformer and the N-1 guideline,is defective without considering tie-line capacity and the main transformer’s short-term overload.In this paper,an improved method was proposed to solve this problem.Firstly,the tie-line constraint matrix was established to accommodate the large differences of tie-line capacities among transformer substations;then,a virtual contact matrix was defined to describe the load’s second transfer process caused by the main transformer’s short-term overload,and on which basis formed the load synthetic transfer matrix.These two modifications were introduced into the traditional method,to calculate the power supply capability of distribution system considering actual operating conditions.Finally the effectiveness of the method was verified through analyzing the power supply capability of a real distribution system.