【Objective】Soil detachment is the initial stage of soil erosion, providing sediments for subsequent transport and deposition. Yet, it is governed by dynamics of the flow on slope, soil properties, and growth of the vegetation root system. During the season when vegetation grows, the growing root system may cause changes in soil properties and the seasonal variation of soil properties may affect soil detachment processes, which has been reported in just a few of the researches on loess in Europe. Most of the researches in the past used to focus on soil detachment on gentle slopes, leaving mechanisms affecting soil detachment processes on steep slopes still in haze. In hilly regions of the Loess Plateau, the growth of grass roots in rehabilitated grasslands under the practice of “Grain for Green” was found to have caused changes in soil properties of the lands and consequently in soil detachment processes. However, it is still unclear how they are related to each other quantitatively. 【Method】In this study, an experiment was carried out using water flumes with adjustable bed gradient to investigate changes in soil detachment processes during the plant growing season and their potential affecting factors and especially effects of changes in soil properties and growth of grass roots on soil detachment capacity in two tracts of rehabilitated grasslands different in “Grain for Green” history. One (Tract A) had been planted with Leymus secalinus for 3 years and the other (Tract B) with Medicago sativa for 17 years. 【Result】Results show that soil detachment capacity of the two tracts of grasslands declined significantly during the whole growing season (p＜0.05). The mean soil detachment capacity was much higher in Tract A (0.076±0.036 kg m^-2 s^-1) than that (0.057±0.055 kg m^-2 s^-1) in Tract B. They varied seasonally and were mainly affected by hardening of the soil, formation of water-stable aggregates, and root growth. With increasing soil cohesion, bulk density, content of water-stable aggregates, and grass root density, soil detachment capacity in either soil declined exponentially. Seasonal variation of the soil detachment capacities of two soils could well be simulated with soil bulk density, root density, and flow shear stress (R2＞0.73，ME＞0.85). Dynamic changes of the soil properties, such as soil cohesion, bulk density and water-stable aggregates, and growth of the grass roots are the main factors that affect the seasonal variation of soil detachment capacity in the two tracts of rehabilitated grasslands in hilly regions of the Loess Plateau of China. The mean soil detachment capacity of Tract A and Tract B during the whole plant growing season was only 6.32% and 4.79% of that of the control tract, respectively, which suggests that the practice of “Grain for Green” in hilly regions of the Loess Plateau of China is conducive to soil and water conservation in that area. 【Conclusion】 The findings of this study may serve as theoretic basis and data support in elaborating hydrodynamic mechanisms of the soil detachment processes, evaluating effect of grass roots in fields of “Grain for Green” on soil and water conservation, and rationalizing arrangement of soil and water conservation practices and measures in hilly regions of the Loess Plateau of China.