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In order to study and illuminate the effects of different raindrops and runoff characteristics on size-selectivity of sediments crumb structure detachment and transportation in soil erosion process, a rainfall simulation study was conducted to examine the sediment particle dynamic distribution characteristics in Beijing, China. Erosion plots (5m×1m) were installed in a 20° hillslope. Experiment treatments included three rainfall intensities (35mm/h, 65mm/h and 100mm/h) of representative rainfalls and three average vegetation coverages (0%, 30% and 80%). The changes of sediment crumb structure at erosion plots with and without raindrop impact were observed through placing the nylon net over the soil surface. The results showed that sediment concentration and soil loss were reduced by 25.91%~31.15% and 35.10%~41.20%, respectively, after eliminating raindrop impact. The sediment mean particle size (D50) of the slope without raindrops was less than that of the slope with raindrops, which was decreased by 23.47%, 27.17% and 31.63% under the three kinds of rainfall intensities. At the initial stage of runoff, coarse sand-size, fine sand-size, fine silt-size and clay-size existed in the form of aggregation, and coarse silt-size was in a state of primary particles. With the extension of rainfall, the E/U (effective-ultimate size) ratio of each graded size was gradually tended to be the value of 1, which indicated sediment particles existed in the form of primary particles. The correlation analysis of the sediment particle size and water flow power under different vegetation coverages showed that the slope erosion process was closely related to the hydrodynamic characteristics. The effect of splash on soil surface accounted for 28.09% of the total detachment degree before the runoff generation, and the soil aggregate fractal dimension increment with no raindrops on the slope was 48.43% of that with raindrops on the slope. In the sediment transport process, sand particles was decomposed into small particles, coarse silt particles were transported as primary particles, and clay particles was gradually enriched in the erosion process. The sediment size-selectivity characteristics by erosion dynamics in the process of erosion were revealed, which deepened the research on erosion regularity of different erosion processes.