Taking slope research as a breakthrough point, according to the variation law of runoff and sediment in the middle reaches of Yellow River and on the basis of cause and effect analysis, proposes "calculating sediment by flood" method, and apply this method to the slope measures analysis of the south watershed between Hekou and Longmen. The results show that the method is relatively reasonable, this study direction is a kind of useful exploration to methodology and has broad research prospects for the theory and practice.