The calculation of supply capability for a power distribution network which takes the N-1 criterion into account needs to consider both the power supply capability of the substations and the transfer capability between substations. Given the characteristics of “closed-loop design and open-loop operation” of the distribution network, a directed graph is used to describe the connection relationship between substations and calculate total supply capability. First, the concept of ideal ‘one supply, one backup’ structure and its directed graph form are introduced. It is pointed out that the connected feeders can be split into one supply one backup structures according to the transfer mode. Then, based on the actual distribution network operation requirements, the connected feeders are decomposed according to different connection modes, thereby decoupling the feeder capability constraints during load transfer. This simplifies the complex distribution network into a clear and intuitive directed topology diagram. Finally, the calculation model of total supply capability is established based on the distribution network described by the directed graph. The practicality and effectiveness of the method are illustrated by an example. This work is supported by National Key Research and Development Program of China (No. 2017YFB0902800).