采用多谱勒三相流速仪，在长江干流黄陵庙水文观测断面进行脉动流速观测，获得了不同流量级、不同垂线的脉动流速资料。在普朗特掺长假定的基础上对掺长与水深的函数关系进行分析研究，目的是寻求雷诺应力计算的可行途径，并为天然河流雷诺方程的封闭探求方法。提出了计算掺长的拟合多项式，基于提出的拟合多项式对雷诺应力进行计算，计算值与实测资料吻合良好。

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. 