[Objective] Once surface runoff gets coupled with subsurface water flow, severe soil erosion would be triggered off. As one of the key parameters of soil erosion, sediment transport capacity is of important theoretical significance to perfecting the knowledge about soil erosion process as affected by subsurface water flow.[Method] With the aid of certain experimental methods and devices, an experiment was designed and carried out with nozzles laid out at different locations under an artificial slope of purple soil to supply water from the bottom, so as to simulate subsurface water flow. Through a series of tests, sediment transport capacity of rill flow was determined as affected by water saturation depth (5, 10 and 15 cm) in the subsurface, flow rate (2, 4 and 8 L·min^-1) and slope gradient (5°, 10° and 15°). A multivariate nonlinear equation was used to analyze effects of flow discharge, slope gradient, subsurface water saturation depth and their interaction on sediment transport capacity.[Result] Experimental results show that the maximum volume of sediment increased with increasing subsurface water saturation depth, but it increased more when the saturation depth reached 5-10 cm than when it did 10-15 cm. When the subsurface water saturation depth increased from 5 to 15 cm, the increment rate of rill sediment transport capacity varied from -6.67% to 43.24% on the slope of 5°, from 2.22% to 15.37% on the slope of 10°, and from 0.99% to 11.54% on the slope of 15°, which indicate that rill sediment transport capacity increased as subsurface saturation depth increased, but with the increment rate declining and gradually leveling off in the end. And the determination coefficient of the equation fitting sediment transport capacity with subsurface water saturation depth was 0.87-0.99, but when the flow discharge was 2 L·min^-1 on the slope of 5°, the determination coefficient was only 0.35, because there was no interaction between subsurface water flow and surface runoff. Moreover, a nonlinear correlation equation was established for fitting relationships between slope gradient, flow discharge, subsurface water saturation depth and sediment transport capacity with a determination coefficient of 0.98. Compared with the slope gradient, the flow discharge was a more important factor affecting sediment transport capacity.[Conclusion] Sediment transport capacity is positively related to slope gradient, flow discharge and subsurface water saturation depth, but as it is rising on, it is losing its momentum gradually. Subsurface water saturation depth is an important factor affecting sediment transport capacity, exhibiting an exponential relationship. With the presence of subsurface water flow, flow discharge and slope gradient interacts significantly with each other, and subsurface water saturation depth and flow discharge does too. All the findings in this experiment may provide a certain theoretical and scientific basis for elucidating the mechanism of the coupling of surface runoff and subsurface water flow affecting soil erosion. Therefore, in the study on erosion control on slope farmlands of purple soil and related soil erosion prediction models, it is essential to recognize the importance of subsurface water flow in the effort to improve rationality of the prevention and control measures and accuracy of the prediction models.