A rainfall simulation study was conducted to examine the sediment particle dynamic distribution characteristics in splash and sheetwash process. In order to reveal the surface soil aggregate mechanical breakdown, the experimental treatments included three rainfall intensities (30, 60and 90mm/h) of representative rainfalls, two slope gradient (5°、15°) and three average vegetation coverage (0%, 30%, 75%). In our experiment, sediment samples detached by raindrops splash and transported by runoff flow were collected separately. The results showed that sediment particle size mainly concentrated in the 0.1~0.05 mm and 0.05~0.02 mm. For the splash sediment, sand particlesS(2-0.05 mm) content is higher than sheetwash sediment, and silt (0.05-0.02 mm) and clay particles (<0.002 mm) is lower. The MWD value of spalshed is greater than the MWD value of washed. With regard to sediment size variation characteristics in erosion process, the coarse sand (2-0.25 mm) and fine sand (0.25-0.05 mm) decreased and silt (0.05-0.02 mm) and clay particles (<0.002 mm) increased with rainfall duration to sediment particles by splashed. The sheetwash sediment particles size mainly includes silt (0.05-0.02 mm) and clay particles (<0.002 mm).The relationship of the each particles size by splashed and washed with slope runoff flow and water depth indicates slope erosion process is closely related to the hydrodynamic factors. Meanwhile, the fractal dimension of sediment particles size distribution (D) and relative mechanical breakdown index (RMI) of soil aggregates has better fitting, and reflection mechanical breakdown degree of slope splash and sheetwash sediment particles with rainfall duration. The experiment results can provide a theoretical reference for agricultural water-soil engineering and slope erosion process prediction model of Rocky Mountain Area.