In order to study the law of rill erosion on the loess slope, and explore the indoor calculation method for the detachment capacity of water flow, this paper took loess soil as the research object, four flow rates (2, 4, 6, 8 L/min), and four slope gradients (5°, 10°, 15° and 20°) were set up, and the length of soil trough was 12 m. The critical rill length and sediment transport capacity of loess soil slope were obtained. Based on the functional relationship between the above two factors, the calculation formula of detachment capacity was deduced. According to this, the change rules of critical ditch length, sediment transport capacity and detachment capacity under different test conditions were analyzed, and the accuracy of this test method was verified. The results showed that under the designed hydraulic conditions, the critical rill length of rill erosion on loess slope varied from 5.33 to 11.12 m, and the critical rill length decreased with the increase of flow rates and slopes, and the sediment transport capacity increased with the increase of flow rates and slopes. There was an obvious linear relationship between detachment capacity and flow rate, and a logarithmic relationship between detachment capacity and slope. Compared with other methods, this test method had the advantages of convenient operation and high coincidence of results, and could better determine the denudation ability of rill erosion in loess area. The research results can further improve the theory of rill erosion on the loess slope.