Taking Liutonghe tributary within the Xiluodu Reservoir for example, we study the determination method of navigable waters, waterway classification and design navigable stage within the high dam.Firstly, we demonstrate the starting point of the tributary waterway by analyzing the backwater length, water stage guaranteed rate, channel conditions, distribution of mineral resources, port planning, etc. Secondly, we determine the waterway classification of the tributary by analyzing channel conditions, navigable clearance dimensions of river crossing building，waterway classification of main stream, etc.Finally, we determine the design lowest navigable stage by calculating in sections. The design lowest navigable stage of perennial backwater area adopts the level of dead water.The design lowest navigable stage of fluctuating backwater area is comprehensively analyzed according to waterway regulation measures feasibility, waterway regulation measures investment, navigational guaranteed rate, and navigation capacity.