In order to understand the relation between the developing process of rill erosion and flow shear stress, the paper simulated the developing process of rill erosion applying hydrodynamics software ANSYS CFX (version 15.0.7) to explore the distribution of flow velocity and shearing stress in the rill section. Based on flume experiment, 6m long flush tank was used and rill development in 8 sections was measured. The experimental flow rate was 800L/h. The paper anaylized the characteristics of flow velocity and sheer stress combined with the measured value of rill section and it is a supplement in computing hydrodynamics of the fracture mechanism of rills under water flow. The results showed that the rill section average flow velocity was 0.71 times of the surface velocity, the correction coefficient was 0.78 with the dye method to determine the rill flow velocity when using the way of CFX numerical simulate pattern method. Rill section shape affects the magnitude of shear stress. The sheer stress value of broadshallow section was less than the narrowdeep section which is between 1.41~2.10Pa. The shear stress determined the rill development, and the point sheer stress distribution of rill section was normal distribution. The point sheer stress of rill section was consistent with the rill development and sheer stress was tightly correlated with the development of each point in the section.