Quantitative simulation on soil detachment rate is the base of process-based soil erosion model development. This study was conducted to analysis hydro-dynamic mechanism of soil detachment process for original soil core in loess region under the large range of flow discharge (0.5~2.0 L/s) and slope gradient (8.8%~46.6%). The results indicated that detachment rate of disturbed soil is more greater than that of original undisturbed soil caused by the disturbance of soil structure. It is necessary to use original undisturbed soil to investigate the mechanism of soil detachment and soil erosion. Soil detachment rate increased with both flow discharge and slope gradient increasing. However, the regression equations were slightly different between each other. Soil detachment rate can be simulated with power function of flow discharge and slope gradient (R~2=0.95). Due to the influence of sediment transport and soil sample disturbance, erodibility parameter of current study was significantly different with the relative results obtained from disturbed soil samples. Therefore, it is necessary to use original soil sample to simulate soil erosion mechanism. Among three current used hydro-dynamic parameters of shear stress, unit stream power, and stream power, there was closely relationship between soil detachment rate and stream power.