To define the rill erosion characteristics and the law of runoff and sediment yield of loess soil in the process of rill development under runoff scouring, rill simulation test was conducted in variable slope soil troughs, and three flows (2, 4 and 8 L/min) and four slope gradients (5°, 10°, 15° and 20°) were set up. The maximum runoff sediment concentrations during rill development on the loess soil slope under different slopes and flows were measured, then its corresponding sediment transport capacity (A) was obtained. The results showed that, when the slope was constant, the sediment transport capacity increased linearly with the increasing of the flow rate, and the steeper the slope was, the more obvious the increase was. When the flow rate was low, the sediment transport capacity increased slowly with the increasing of the slope gradient. When the flow rate reached 8 L/min, the sediment transport capacity increased faster with the increasing of slope gradient, but it hardly changed after the slope increased to 15°, indicating that flow rate had more significant effect on sediment transport capacity. The trend of sediment concentration (c) with length of rill (x) conformed to mathematical formula c=A(1-e-Bx). We run a simulation by scouring the slope surface of 1 m and 2 m under all combinations of different slopes and flow rates we had set up, and the sediment concentration of runoff measured by scouring and the sediment transport capacity (A) under each combination were substituted into the formula, and attenuation coefficient (B) of sediment concentration varying with the rill length under different test conditions was calculated by method of undetermined coefficient. The results could provide theoretical and scientific basis for soil and water conservation research and practice in loess soil.