Large number of production and construction projects disturb the surface and destroy the original surface vegetation with the rapid development of economy and society and the rapid increase of engineering construction projects. Large-scale and unbalanced excavation and filling make excess soil and rock mixed and accumulated. The excess soil-rock mixed media formed in the construction of mountain projects are accumulated in the upstream and slope of ditches and rivers, which is prone to serious soil and water loss under heavy rainfall conditions, and even cause disasters such as landslides and debris flows. This study took the artificially remolded spoil heaps (with a slope degree of 25°) as the research object, and carried out the research on the influence of slope length (3 m, 5 m, 6.5 m, 12 m) and gravel content (0, 10%, 20%, 30%) on the spatial distribution characteristics of velocity and sediment yield characteristics of spoil heaps under extreme rainfall conditions (with a rainfall intensity of 2.5 mm/min) through the simulated rainfall test. Results showed:(1) The flow velocity was in a fluctuating state during the whole rainfall process, and the fluctuation amplitude decreased with the increase of gravel content and mass fraction under slope length by 12 m. However, the flow velocity under the slope length of 3 m, 5 m and 6.5 m increased at first and then tends to be stable with the runoff duration. The average velocity of the middle and lower slope was 1.12 to 1.54 times that of the middle and upper slope. With the increased of the slope length, the average velocity of the flow increased by 17.81% to 335.94%, and the average velocity of spoil heaps with gravel was 22.88% to 54.67% lower than that of the pure spoil heaps. The mass fraction of gravel had more significant effect on the flow velocity than the slope length. (2) The average soil loss rate of pure spoil heaps increased significantly with the slope length, with an increment of 7.88% to 87.67%. However, the gravel-bearing spoil heaps decreased with the increasing of slope length from 3 to 5 m (decreased by 9.10% to 28.86%), but then increased significantly. There is a critical slope length. The average soil loss rate of spoil heaps with slope length of 6.5 m and 12 m is 22.33% to 122.48% higher than that of slope length of 3 m. Gravel could significantly reduce the slope erosion of spoil heaps, and its sediment reduction efficiency reached 24.99% to 59.54%. The contribution rate of gravel to the average soil loss rate of spoil heaps was 1.13 times of the slope length. (3) The impact of flow velocity on the soil loss rate in the upper and middle slope of the spoil heaps was greater than that in the lower and middle slope. There was a significant correlation between the average soil loss rate and flow velocity. The flow velocity could be used as a characteristic parameter of the average soil loss rate, and its linear fitting relationship (R² was 0.67 to 0.95). The results further deepen the connotation of soil erosion discipline, which can provide data basis for the prediction model of spoil heaps erosion of construction production and construction projects, and also provide theoretical basis for clarifying the internal mechanism of engineering disturbance slope erosion.