【Objective】 Soil detachment rate refers to the quantity of soil detached by water flow in a unit of time and/or area. Quantitative analyses of the soil detachment processes in different soil layers in granite collapse regions is of great theoretical and practical significance to accurate prediction of soil detachment processes and construction of a physical model for erosion of collapsed hills. 【Method】 Collapsing hill erosion is a speci?c type of soil erosion in hilly granitic regions of tropical and subtropical South China, and may result in extremely rapid water and soil loss. Knowledge of how collapsing hill erosion affects the soil physical and chemical properties of different soil layers in the region is important to understanding the evolution of soil quality. In this study, an in-lab runoff scouring experiment was conducted on artificial slopes of top soil layer, red soil layer, sandy soil layer and detritus layer developed in the collapsed granite hills in Tongcheng, Hubei Province. The runoff scouring experiment had the slopes set at 8.8%, 17.6%, 26.8%, 36.4% and 46.6% in gradient and scoured with flow varying in rate (0.2 L s^-1, 0.4 L s^-1, 0.6 L s^-1, 0.8 L s^-1 and 1.0 L s^-1), separately for cross checking . The artificial slope was prepared on a steel trough 0.2m high on both side, 0.2 m wide and 3.8 m long and on the top of the trough was a water tank, 0.2m high, 0.4 m wide and 0.2m long, filled with water to supply water flow steadily at a required rate. The experiment was to investigate relationships of soil detachment rate with various hydrodynamics parameters in four different soil layers and to explore hydrodynamic mechanisms of the erosion in collapsing granite hills. 【Result】 Results show that under the condition of a certain slope, soil detachment rate increased with increasing runoff flow rate, and varied sharply with soil layer; the highest detachment rate was found in the detritus layer, which was followed by sandy soil layer, red soil layer and top soil layer; under the same runoff flow rate, soil detachment rate decreased with the scouring going on and tended to level off, regardless of type of soil layers. Besides, the complex effect of slope degree and flow rate on detachment rates in all soil layers could be well fitted with the two-variable power function equation (R² > 0.878). The effects of shear stress and stream power of the runoff on detachment rate on all soil layers could be well described with the linear equation (R² > 0.926), yielding a correlation coefficient much higher than that of the fitting of unit stream power with the polynomial equation (R² < 0.830). Therefore, shear stress and stream power of the runoff can be used as hydrodynamic parameters to describe the soil erosion in different soil layers of collapsed hills. The threshold of runoff shear stress varied with soil layer in a decreasing order of top soil layer > red soil layer > sandy soil layer > detritus layer or 0.28 Pa > 0.13 Pa > 0.10 Pa > 0.07 Pa. 【Conclusion】 The detritus layer is the highest in erodibility, followed by the sandy soil layer, red soil layer and top soil layer. Hence, the resistance of the soil layers against runoff erosion gradually decreases with soil depth as the soil layers exist vertically in the soil profile in the collapsing hill region.