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Critical hydraulic characteristics of soil detachability are essential factors for soil erosion predicting. There is still insufficient knowledge on key factors and governing mechanism of soil detachability, especially for various near-surface soil water conditions. The typical eroded yellow soil area in the upper and middle Yangtze River was taken as the research area. In order to illustrate the response of critical condition of soil detachment under different subsurface soil water conditions, scour flume was used to measure hydrodynamics parameters. Five antecedent soil moisture contents (5%~23%) and five slope gradients (1.0°~10.0°) were designed. Results showed that critical flow velocity, flow depth and flow pattern of soil detachment all showed power function decreasing trends with the increase of slope gradient and antecedent soil moisture content. When the slope gradient was smaller than 5.0°, critical hydrodynamics parameters of soil detachment were influenced by coupling effects of slope gradient and antecedent soil moisture content;otherwise, it would be mainly influenced by slope gradient. It was also proposed that the simplified power function equation could be used accurately when slope gradient was larger than 5.0°. Most of the flow patterns belonged to laminar flow and subcritical flow. The slope gradient and antecedent soil moisture content played an important role in the overland flow Darcy-Weisbach friction factor. When Reynolds number and critical discharge per unit width were increased, the Darcy-Weisbach friction factor was decreased as power function.