To identify the major elements reducing the total supply capability（TSC）of distribution networks，a family of hierarchical TSC indices is established.The feeder level TSC，transformer level TSC，TSC jointly conditioned by feeders and transformers，the second transmission line level and comprehensive TSC are defined，respectively.A set of matching indices are defined to identify the matching degree of the TSC of the devices in adjacent levels.A series of indices of fullness degree are also defined to represent the difference between the actual load and the load in the ideal condition with maximum TSC on devices.The methodology for finding the factors reducing the TSC of distribution networks based on the proposed indices is described.An example is given to demonstrate the proposed approach in detail showing its feasibility and effectiveness.