Abstract: Reduction effects of grass on runoff and sediment as well as its mechanical mechanism was studied under rainfall intensities of 45,87 and 127 mm/h on 20°grass slope using simulated rainfall experiment.The results showed that soil infiltration rate of grass slope was 2.1-4.2 times higher than that of bare slope.Flow velocity,runoff and sediment amount on grass slope decreased by 77.3%-79.8%,51.9%-99.1% and 93.6%-99.2% respectively compared with those on bare slope.Relationship model between flow shear stress and sediment amount was built by analyzing of carrying force of soil partial,which can be used to predict the amount of soil loss on grass slope.The critical flow shear stress of 2.857 N/m² on grass slope and 0.861 N/m² on bare slope were measured based on the relationship model under experiment condition.The sediment amount ncreased with the increase of flow shear stress.The experiment results are meaningful for quantifying runoff and sediment reduction and deepening soil erosion mechanical process also.