Engineering piles is a special man-made geomorphic unit and has been found much more serious in soil erosion. The studying of hydrodynamic process in the engineering piles is of great necessity, for the engineering piles with three-dimensional is quite different from the traditional slope. In this paper, a self-made platform was used to simulate the three-dimensional pyramidal engineering piles and an indoor artificially simulated rainfall experiment was carried out under different rainfall intensities (1.0, 1.5, 2.0, 2.5 mm/min) to study the hydrodynamic process and shape characteristics of rills of engineering piles with different mass percentage of gravel (0, 10%, 20%, 30%, 40%). The results showed that: (1) Flow velocity and runoff intensity increased with the increase of rain intensities and decreased with the increase of gravel contents. Both the rainfall intensity and gravel contents had the significant influences on flow velocity and runoff intensity, and the influence of rainfall intensity was greater. (2) The runoff was in the laminar and subcritical flow state, and Reynolds number and Froude number increased with the increase of rain intensities and decreased with the increase of gravel contents. (3) Erosion detachment rate increased exponentially with the increase of rainfall intensities, and decreased linearly with the increase of gravel contents, there were significant power function relationships between erosion detachment rate and shear stress, stream power, unit flow power and unit energy of water-carrying, among which stream power had the best correlationship, indicating it was the optimal factor to describe soil erosion. (4) The rill occurring time advanced and the length, width, depth of rill increased with rainfall intensities increasing, while the rill occurring time was delayed, the depth and width of rill decreased gradually with the increase of gravel contents under the same rainfall intensity.