Abstract: Most algorithms of valley-line extraction from Grid-DEM are based on the natural characteristics of surface runoff.A key parameter of these methods is the threshold of flow accumulation.At present the choice of the threshold is usually subjective and random,not based on a quantitative model.In this paper,we study the relationship of flow accumulation with the average bifurcation ratio of valley-line network in the context of multi-scale based on the self-similarity of the valley system.The effective threshold range is determined based on the principle of self-similarity and the average bifurcation ratio closer to constant.