The coarse sediment source area with high concentration of middle Yellow River Basin is the most strong erosion area of the Loess Plateau, and is a main source of coarse suspended sediments which led strong sedimentation at the lower Yellow River. The soil erosion in this area is influenced by multiple factors. Vegetation as a main factor has been paid close attention. In this work the spatial distribution of the erosion modulus, forest cover and total forest and grass cover was discussed. Some non-linear relationship between some high erosion modulus and vegetation cover seems existent. The high erosion modulus was defined as the extreme erosion modulus: the maximum erosion modulus under a similar vegetation cover condition. From the relationships between extreme erosion modulus and vegetation cover, the threshold value of 24.2% for the total forest and grass cover, and 12% for the forest cover was obtained. It means that, if the other multiple factors do not change remarkably, the extreme erosion modulus increases with the increase of forest cover or total forest and grass cover when the forest cover is lower than 12% or the total forest and grass cover is lower than 24.2 %, then it decreases with the increase of the covers. This result may be helped us to manage the middle Yellow River basin efficiently.