River flow is affected by boundary shear stress, and too large boundary shear stress will lead to erosion and reduce the stability of river banks, and cause river morphology changes by affecting sediment deposition and transport. To study the shear stress of river boundary is equivalent to the study of flow characteristics in river. Because there are too many complex factors affecting the boundary shear stress in rivers, a two-dimensional rectangular straight open channel flow model was established by numerical software after removing all factors, and the laboratory experiments were carried out on the rectangular straight open channel model by using similarity theory. Firstly, numerical simulation was carried out for flows in uniform straight channels of 5 different scales. Since the stage-discharge relation obtained from the simulation results is highly consistent with the stage-discharge relation deduced by Manning formula, it can be considered that the parameters simulated by the model are true and reliable. In the case of non-uniform flow, through the concepts of under-uniform flow and super-uniform flow, 9 cases of straight open channels under different discharges were simulated in this paper. The momentum theorem was used to analyze the force at 101 points with the same spacing in the center of the non-uniform straight path. The average value of shear stress was taken between every two points, and then the resistance of all points was summed up to approximate the integral of the river bed resistance, which will greatly reduce the influence of the error caused by uncertain factors such as stage fluctuation. A general formula of bed shear stress was derived, which showed that the bed shear stress could be calculated by the water depth h, flow velocity u and Manning roughness coefficient n, regardless of whether the flow of rectangular open channel is uniform or not. To verify the new formula, the new formula was compared to four existing formulas, and the results showed that the new formula conformed to the momentum theorem and had physical significance. The research result is simpler than the traditional methods and can provide a faster algorithm in required calculations.bm