In the numerical simulation of fluid flow and sedimentation in a natural river, the simulation precision is often referred to as a standard to judge the quality of a model. The factors that affect the models simulation precision are represented by rational assignment of momentum transfer coefficient and turbulent diffusion coefficient. On the basis of Prandtls mixing length and the momentum transfer theory, the Reynolds stress and time averaged velocity gradient are combined. Utilizing the field measured data obtained under the conditions of different discharges, vertical fluctuating velocities and the distribution of suspended sediment concentration along vertical lines at the Huanglingmiao hydrologic observation cross section on Yangtze River, the momentum transfer coefficient and small scale turbulent diffusion coefficient are calculated and their relationships analyzed.