A novel polyurethane (W-OH), namely an eco-friendly hydrophilic polymer, has been widely applied in the field of soil erosion. However, recent research has not revealed the process and mechanisms through which W-OH application influences the soil detachment by concentrated overland flow (hereinafter referred to as soil detachment). In this study, the effects of the W-OH concentration on the physical and mechanical properties and the detachment capacity of colluvial deposit slope soil were investigated, and the impact of the relationship between the flow discharge and the W-OH concentration on the soil detachment capacity was examined under the experimental conditions. The results indicated that W-OH application significantly increased the large-particle content in the soil samples, enhanced the strength properties of the soil samples, reduced their separation capacity, and increased their stability. The structural equation modelling results revealed that W-OH application influences the soil detachment capacity primarily by affecting the shear strength, which exerts a significant negative effect on the detachment capacity (path coefficient = -0.57, p < 0.001). The soil detachment capacity prediction equation, which is based on the flow discharge and W-OH concentration, exhibited satisfactory accuracy (Nash-Sutcliffe efficiency (NSE) = 0.964) and can be used to predict the soil detachment capacity with high precision under similar experimental conditions. In addition, at a W-OH concentration above 1.53%, the impact on the soil detachment capacity is greater than that of the flow discharge. This study focused on investigating the process and mechanisms through which W-OH application reduces soil erosion on colluvial deposit slopes, thereby providing reference data for the management of Benggang erosion.