The determination of the velocity in rill flow is the basis of studying the soil erosion. The velocity of rill flow is influenced by flow discharge, sediment concentration, underlying soil and so on. Under the steady discharge, by assuming the resistance of viscosity is directly proportional to the velocity of flow, the dynamic equation of rill flow is deduced. By nonlinear simulation of the experiment data, we obtained functions between velocity of flow and time, the friction coefficient and erosion intensity, and the viscosity force and sediment concentration in flow. The simulating procedure and results show that the model is feasible when the velocity of flow is lower. Velocity of flow is mainly affected by slope and sediment concentration. When the slope is steeper, the velocity enlarges more quickly, but needs more time to achieve steady value.