The velocity of slope runoff flow is an important parameter to evaluate soil erosion, and it is also a key factor in the soil erosion forecast. A runoff flow velocity measurement system based on LabVIEW was developed, which consisted of test-bed, photoelectric sensor, data acquisition card, measuring and controlling software. The experiment was completed in room simulation test-bed using foam particle as a tracer. The result indicates that the test-bed can recycle water and soil. The system measures runoff flow velocity accurately at the scope of 0～400 kg/m³ and 0～25° of slope gradient. It is found that the greatest relative error of the system is 2.29%. This measurement system provides a new, valid method for speedy and accurate measurement of runoff flow velocity.