[Background] The rill erosion is the main source of the total erosion amount, and it is the greatest contribution to the slope erosion. Rill flow velocity changes with the slope gradient changing. The sediment load changes not only by the impact of soil properties but also by the hydraulic characteristics and slope characteristics. At present, soil erosion study for purple soil was relatively shortened and the data that could be used is very little. [Methods] A series of flume experiments of restrictive rills were conducted to understand erosion characteristics of rills under the condition of different slope gradients and slope lengths. Quantitative study of the rill erosion is the basis for best understanding the slope erosion mechanism. The soil used was purple soil, a clay soil. The test sediment was air dried and sieved through a 5 mm sieve. Three slope gradients: 5°, 15°, and 23°, as well as three flow discharges: 5 L/min, 15 L/ min, and 25 L/ min were adopted in the experiments. Flow discharge was controlled by a series of valves installed on a flow diversion box and measured directly by a calibrated flow meter. The elevation of the upper flume end was adjusted by a stepping motor, allowing adjustment of the bed gradient up to 45%. The rills were 10 cm wide. Rill length was 0.5 m, 1 m, 2 m, 3 m, 4 m, 5 m, 6 m, 7 m, 8 m, 9 m, and 10 m respectively. [Results] The velocity of clean water in rill was larger than that of muddy water under the same slope gradient and runoff. Both slope gradient and runoff discharge impacted the flow velocity. Under different runoff discharge, the flow velocity of slope 5°had little variation and was stable. The flow velocity of 15°and 23°decreased with the slope length increasing, and the situation tended towards stability. Under a given slope gradient and runoff discharge, the sediment load increased as the slope length increasing, but the rate of increasing was getting smaller and smaller till it approached a stable value. On the slope gradient of 5°, the slope length value while sediment load reached stable became shorter along with the increase of flow discharge, and the values were 7 m, 5 m, and 5 m respectively at the flow discharge of 5 L/ min, 15 L/ min, and 25 L/ min. On the slope gradient of 15°and 23°, the slope length values while sediment load became stable were 7 m and 6 m under the flow discharge of 5 L/ min and 15 L/ min. The sediment load did not become stable when the slope length was 10 m that was the largest length of the experiment under the flow discharge of 25 L/ min. The yield of sediment and slope length presented a relationship of logarithmic function. [Conclusions]The results are conducive to the comprehensive understanding rill erosion of purple soil and provide basic data for physical erosion model's establishment.