“The distribution of shear stress on the bed surface of the sharp bend river section is the key to controlling the erosion and deposition pattern of the bend. However, there is still a lack of quantitative research on how the coupling of secondary flow in curved sections and mainstream shapes the spatial distribution of bed shear stress, and which processes can be ignored at the engineering scale. This article combines high-precision 3D experiments and numerical simulation results to decompose and evaluate the magnitude of the average tangential and radial momentum equations of water depth in a cylindrical coordinate system. The analysis shows that the main control mechanism of bed shear stress comes from the coupling of mainstream transport and circulation transport: in the tangential momentum balance, the mainstream transport term and circulation transport term dominate for a long time, and the longitudinal water surface slope only shows local enhancement near the geometric curvature mutation; The radial momentum balance is mainly determined by the radial transport term and the transverse water surface gradient term, followed by the circulation transport term. The Reynolds stress term in both sets of equations is always at the minimum level, indicating that the bed shear stress does not depend on the contribution of the turbulence term under the working conditions in this paper. Within the low Froude number range of Fr=0.12~0.21, the relative magnitude sequences of each physical term remain stable and only amplify synchronously with the flow conditions, indicating that the distribution of bed shear stress is insensitive to changes in Fr. In addition, for the sharp bends with large H/R studied in this article, the coupling effect of secondary circulation and surface separation flow will form a narrow and strong high shear stress band near the convex bank in the 30 °~90 ° bend section, with a peak value about twice that of the surrounding area; If the two-dimensional model still uses the bed constant resistance coefficient, it will systematically underestimate the shear stress in the danger zone and overestimate the longitudinal average flow velocity. Based on momentum analysis, this article proposes that the resistance coefficient should gradually change with H/R (combined with relevant turbulence intensity), providing a quantitative basis for the two-dimensional simulation correction, revetment layout, and river regulation design of sharp bend river sections.”