A water–soil coupling model of channel fluid movement is established considering the interaction of gully bed erosion, slope confluence, and rainfall, which can provide theoretical basis for watershed risk assessment, disaster prevention and mitigation, and identification of potential debris flow gully. By analyzing the erosion process of the channel fluid on the movable solid source of the gully bed, and coupling it with the slope confluence and the rainfall over the channel with temporal and spatial variability, the water–soil coupling model of the channel fluid movement in the small watershed can be established. The finite difference method was used to discretize the water–soil coupling model of fluid movement in time and space. A computer code for the coupling model of fluid motion was written in MATLAB. Meanwhile, laboratory experiments were carried out with varying flow rates and groove slopes to observe the evolution of fluid flow depth, flow velocity and fluid bulk density under different working conditions. By analyzing the results of the 12 sets of experiments, it was found that under the condition that the slope of the groove remains unchanged, the fluid flow depth and the fluid velocity were positively correlated with the flow and the bulk density was negatively correlated with the flow. At the same time, the numerical simulation results of the fluid flow depth, flow velocity and fluid bulk density at the control point of the groove were compared to and analyzed with the experimental results. It was found that the simulation accuracy of the fluid flow velocity and fluid bulk density were higher than 90%, and the simulation accuracy of the fluid flow depth was higher than 80%. The results revealed that the established water–soil coupling model of the channel fluid movement in the small watershed is accurate.