Abstract: Abstract: The precise study of free surface flows is explored mostly because of their importance in construction of soil erosion models. The accurate and efficient observation methods can lay a solid foundation for the further study of characteristics of slope flow. The objective of this work was to design an experimental system for measuring roll waves. The system was based on a high-precision ultrasonic sensor. The measurement system was composed of ultrasonic sensors, data acquisition box and the computer. The experiment was carried out in a hydraulic flume. The unit discharge varied from 0.167 to 0.500 L/(m·s), and slope gradient was from 3o to 15o. The system was calibrated and the results showed that the average relative error of the measurement system was 0.23%. The average coefficient of variation was 0.66%. The main parameters of roll waves included wave velocity, wave frequency, wave length and wave peak. In this study, by recording roll waves observed in the same section, the relationship between water depth and time was obtained by ultrasonic flowmeter. The average interval time of data acquisition was 15 ms so that the sensor could catch the change of water level in time. Roll wave frequency was obtained by observing the cross section peaks in unit time. Wave velocity could be obtained by two sensors. The two sensors recorded the time that the same roll wave passed through the section and also measured the distance of the two sensors. The wave velocity was calculated as the ratio of distance and duration. The wave length was calculated by the wave velocity and the frequency. The results measured by the proposed system were compared with that obtained from the stylus method and visual method. The results showed that the wave peak measured by the system was mostly similar with that from the stylus method, followed by roll wave frequency, wave velocity and wave lengths. The measuring system had better accuracy and reliability to reach measurement requirements of characteristic parameters of roll waves on slope surface. For the same observation section, the rolling wave was not constant in the measurement process. The wave peak, wave velocity and the frequency had the average coefficient of variation of 0.238, 0.806 and 0.430. The ultrasonic measuring system can continuously work and provide reliable measurements for roll wave parameters. As compared with the conventional method, the measuring system greatly reduced test time and effort to improve test efficiency. The research results have wide application foreground in the improvement of the water flow measurement method on the slope surface.