Rock-bed erosion can be a significant problem in cases where the riverbed contains soft rock or a heavily jointed rock mass. For soft rock without abundant discontinuities, saltation abrasion is often the major mechanism of rock erosion during a large flood with abundant bed load or suspend load. Continuous impacts by particles of bed-load aggregate may result in the accumulative damage of rock material near the surface and cause the broken rock fragments to detach from the intact rock. The rate of erosion is equivalent to the accumulative loss of rock fragments occurring in unit time per unit area. The amount of abrasion can be linked to the dissipated energy during each particle impact. Toward understanding the micromechanics of saltation abrasion, this study aims to employ three dimensional particle flow simulation to model the erosion mechanism of soft rock in microscopic scale. In the numerical simulations, rock material is modelled as a granular assembly with inter-particle bonding. The simulations may be regarded as a “virtual erosion test”; the modelled abrasion process is a result of the particles’ release due to parallel bond break subjected to multiple particle impacts. The extent of damage can be evaluated by the density of broken bonds between particles. By the decomposition of kinetic energy, it appears that the extent of damage and amount of erosion can be correlated to the normal component of kinetic energy. The kinetic energy can be dissipated because of inter-particle bond breakage and slippage. We attempt to use the dissipated energy as a damage index to represent the degree of accumulated damage in the rock material due to continuous particle impacts.