Abstract: A binary tree-based and dyadic indicated codification method is proposed in the digital watershed model(DWM). This method is able to locate any section of drainage network directly and operate the topological relationship effectively. This paper issues the framework and definition of the codification method in the DWM,delineates a drainage network in this special way with its own numerical character. Some detailed arithmetics of the codification method in the DWM is presented in an appendix.