Abstract: To confirm suitable distance between hedgerows for controlling non-point pollution in purple soil area，its control mechanism for the cropland under different distance between L.leucocephala hedgerows of 10° and 15° gradient cropland was analyzed based on hydro-dynamics and observed data of many years in Suining soil and water conservation experimental station．The results indicated that the hedgerows could effectively control soil erosion between hedgerows under the natural precipitation condition if runoff velocity was less than or equal to the start-up velocity of the soil particle(<0.02 mm) while rill erosion was occurring in purple soil area．Through studying the derivation of rill erosion process，it showed that the largest distance between hedgerows for 10° and 15° gradient sloping cropland was 13.73 and 9.00 m respectively. When runoff velocity and start-up velocity of soil particle (<0.02 mm) were in the critical starting condition at the time of rill erosion occurring, the smallest runoff velocity of 10° and 15° gradient slope was 0.183 and 0.190 m/s respectively. It was the main nutrient loss way through soil particle loss on the surface soil, controlling soil particle loss by hedgerow played an important role in the process of non-point pollution control.