Accurate measurement of flow velocity of shallow water on slope is essential to analyze hydraulic parameters and establish soil erosion model. A flume of 12 m long, 0.1 m wide and 0.3 m height was used to measure the velocity of shallow water under five slope gradients (5°, 10°, 15°, 20° and 25°) and four flow discharges (2, 4, 8 and 16 L/min). By recording the time of flow front (front) passing through the flume, the leading edge velocity of flow was calculated. The average surface velocity and average velocity were measured by the dye tracer method and the electrolyte tracer pulse method, and compared with the leading edge velocity. The results showed that the leading edge velocity varied from 0.237 m/s to 1.290 m/s, and it increased with the increasing of flow discharge and slope gradient. The leading edge velocity was more sensitive to flow discharge than to slope gradient. The leading edge velocity could be predicted using a power function of slope gradient and flow discharge. By comparing the leading edge velocity with the average surface velocity measured by the dye tracer method and the average velocity measured by the electrolyte tracer pulse method, the results indicated that the leading edge velocity showed good consistency with the average surface velocity and the average velocity, but the average surface velocity was much larger than the leading edge velocity, and the relative error was -15.018% to -27.825%, and the conversion coefficient between the two velocities was 0.758. The leading edge velocity was relatively close to the average velocity, and the relative error decreased with the increasing of flow discharge and slop gradient, and the conversion coefficient between the two velocities was 0.946. The correction factor was more sensitive to Reynolds number than to flow discharge and slope gradient. The equations established in this study could be used to predict the correction factors. In general, the results of this study could be used as a reference to study the flow velocity of shallow water.