The formulation of circulation velocity by Sun-Dongpo has a concise and reasonable structure,and the results won’t be affected by Karman’s constant and the agreement is acceptable with the experimental findings.But the actual situation of the rough bed can’t be reflected,for the inadequacy in the calculation of transverse slop and the establishment of prandtl’s turbulent stress.For the problems mentioned above,we imported Rozocskii’s formulation of transverse slop which fitted well with the real situation,built the Prandtl’s turbulent stress,and then joined them together to get the improved formulation.Not only did the improved formulation correct the defect in the usual one,but also the results matched very well with the real data,so the improved formulation could be put into use in natural bends.