Field experiments on fluctuation velocities were carried out at the section Huanglingmiao of stem stream of Yangtze River with an Acoustic Doppler Current Profile (ADCP). And a number of data such as velocities of different discharges and different vertical lines were obtained. On the basis of Prandtls mixing length, the function relationship between mixing length and water depth was researched, which searched a feasible way on calculation of Reynolds stress and studied the closed Reynolds equations in the natural river. The paper also puts forward the quasi*.polynomial about calculation of mixing length on which the calculation of Reynolds stress is based. Finally, the calculated values are in good agreement with the field data. 