In the research of the soil erosion and soil losses, the runoff velocity of slope is an indispensable hydrodynamic parameter in the runoff computation and the soil erosion forecast. There is still no special instrument which is widely used to measure runoff velocity. It is very significant to construct a fast measurement instrument on runoff velocity. On the basis of correlation theory, the runoff velocity measurement system was established based on virtual instrument LabVIEW. The system uses the conductance sensor to acquire signal. Effect of the space between conductance sensors on the measurement system and the runoff velocity under five sediment concentrations were studied. Results indicate that the suitable sediment concentration scope of the measurement system is 0～250 kg/m³ and the greatest relative error of the system is 4.5%. While taking the flow velocity measured by the dye tracer technique as standard value to correct the correlation velocity, the greatest relative error of the proved velocity reduces to 3.81%.