In order to traverse all the distribution elements， a composite iterator is formulated based on the distribution network model using the extended common information model（CIM） scheme. In consideration of the presence of many junctions in planning the distribution network in practice， a discussion is made of the development of an improved key nodes based multicast routing algorithm at minimum path cost for solving this problem. The concept of composite-edge is put forward by adjusting the weights of each composite-edge dynamically and the optimal planning result of minimum total cost is obtained. Algorithm and data models are isolated by the iterator， which can effectively reduce the coupling and enhance the maintainability and extensibility. Numerical example results show the feasibility of the distribution planning method proposed.