Water erosion is a time-varying processes controlled by both rainfall and overland flow. A better understanding of dynamic changes in the sediment load and size distribution with various erosive forces can help to develop and verify erosion models. Here, a total of 11 laboratory simulation experiments were conducted in a 1 m by 3 m flume with 3 storm rainfall intensities (60, 90, 120 mm h−1) and 2 inflow rates (5, 7.5 L min−1) on a silty clay red soil. Time-series measurements of the quantity and size distribution of eroded materials were made during 50 min rainfall/inflow time. The mean weight diameter of the effective sediment, sediment enrichment ratio, average stream power and rainfall power were measured and calculated. Fine sediment particles were associated with the short-lived initial stage, which was combined with sheet flow erosion and splash erosion, whereas coarse particles were associated with the rill development and rill stable stages, which were dominated by rill erosion and interrill erosion, respectively. The <0.05 mm aggregates were transported preferentially by the suspension/saltation mechanism, while 0.105–0.25 mm aggregates resisted transportation. Additionally, the sediment load rate showed a significant linear correlation with the stream power at the sheet flow (R2 = 0.84, p < 0.01) and rill development (R2 = 0.72, p < 0.01) stages as well as the rainfall power at the rill development (R2 = 0.76, p < 0.01) and rill stable stages (R2 = 0.78, p < 0.01). Sediment size increased with increasing stream power (p < 0.05) and rainfall power (p < 0.05) only at the rill development stage. The erosion processes play a major role in particle selection, and the erosion forms, aggregate breakdown and soil surface structure development should be considered for a more accurate prediction of size selectivity and the related sediment quality. •Erosion processes were simulated under the combined effect of rainfall and overland flow.•The dynamic changes of sediment size distribution over time were affected by erosion forms.•Aggregates finer than 0.05 mm were transported preferentially by the suspension/saltation mechanism.•The relationships between erosion forces and sediment load changed with erosion process.