The existence of subsurface water flow will aggravate the process of soil erosion. Soil detachment rate is an important physical parameter of the soil erosion prediction model, and the response of detachment rate to subsurface water flow is worthy to study in further. The characteristics of rill detachment of purple soil under different conditions (three subsurface water saturation depths of 5, 10 and 15 cm, three slope gradients of 5°, 10° and 15°, and three flow discharges of 2, 4 and 8 L/min) were studied by using limited rill simulation erosion test. The experimental results showed that the rill detachment rate decreased exponentially and linearly with the gully length and sediment concentration at different saturation depths of subsurface water, respectively. Detachment capacity reached its maximum value at subsurface water saturation depth of 15 cm and decreased with the decreasing of saturation depth of subsurface water. The multiple nonlinear regression equations showed the rill detachment rate was positively correlated with flow discharge, slope gradient and saturation depth of subsurface water, and the standardized coefficients were 1.14, 0.72 and 0.36, respectively, indicating that the rill detachment rate was more sensitive to flow discharge than slope gradient under the condition of subsurface water flow. The study provides some theoretical basis for the rill erosion mechansim under the effect of subsurface water flow in purple soil.