Abstract: Topography is an important factor affecting soil erosion.Distributed soil erosion slope length is an important parameter of topographic factor.The slope length agree with the process of soil erosion, is calculated with the cutoff considered.Slope length, influenced by the terrain, especially the slope changes affects the extraction results are in urgent need of research.In this paper, the mathematical surface and the digital elevation model of the Xiannangou catchment in the Loess Plateau are used as the data source.The length of the slope is extracted by using the LS_TOOL method, and the slope length cutoff results caused by the change in slope are compared and analyzed by principal component analysis.The results show that the greater the down slop decrease, the more obvious the slope cutoff effect.The cutoff factor R₁ and cutoff factor R₂ have positively correlated with the maximum slope length and averages slope length; as there are more steep slopes and channels in the Loess Plateau region, the influence of R₁ is less than that of R₂.When the slope setting of the slope is larger than that of R₁> 0.7 and R₂> 0.5, the change of slope length is obvious.It is suggested that the slope cutoff setting of the slope length of the Loess Plateau region should be R₁=0.7 and R₂=0.5.