Abstract: The overproof sewerage is still a sternly and headachy question in the city drainage system. In order to efficiently and accurately obtain the flow through of wastewater, this paper puts forward an automatic flow direction analysis algorithm based on directed graphs. Firstly, the geometric model of pipeline network, consistency of flow direction and pipeline property are analyzed. Secondly, the algorithm is designed by forward breadth-first search, buffer analysis and search hybrid junction pipeline, realized automatic acquisition of polluted pipelines in the step-search process. Finally, the validity of the algorithm is demonstrated using an example, and comparison with traditional methods, for massive pipeline network data, the proposed algorithm greatly improves the efficiency of analysis.