In order to reveal the sorting characteristics and transportation mechanism of erosion sediment driven by runoff on the steep slope of engineering deposit, the field simulated runoff scouring test under three upper inflows of 10, 20, and 30 L/min were designed, and the particle distribution characteristics of erosion sediment on the steep slope (32°) of the engineering deposit body in Yangling were analyzed. The results showed that the clay and fine silt in the eroded sediment (before dispersion) increased significantly compared with the original soil, which was easy to produce erosion. The fuction of runoff on the fragmentation of aggregates affected the clay content of eroded sediment. The clay content was negatively correlated with the runoff power when the runoff power was less than 1.71 N/ (m·s), but it was positively correlated when runoff power was greater than 3.89 N/(m·s). In sediment, fine and coarse silt particles were mainly transported in the form of single grain, while clay and sand particles were mostly transported in the form of aggregates. In eroded sediment, clay particles were enriched and sand particles were depleted. The particle size of sediment determined its main transportation mode, sediment particles less than 0.11 mm were mainly suspension/saltation transport, and sediment particles more than 0.11 mm were mainly rolling transport. The contribution rate of rolling transport increased first and then decreased with the increasing of runoff transport capacity. The conclusion of this study was conductive to reveal the mechanism of slope water erosion process of engineering deposit body, and could provide scientific basis for improving the prediction accuracy of slope water erosion model on the steep slope of engineering accumulation.