Grain/bed collisions play a significant role in the transportation of granular materials, especially in windblown sand movements. In this study, grain/bed collisions regarding two sand grains simultaneously impact onto a mix-sized sandy bed are numerically investigated by the discrete element method (DEM). Results demonstrate that even under the same initial conditions, the outcome of two sand grains simultaneously impacting a 2D sand packing (termed as two-grain-bed collisions) is different from the superposition of two grain/bed collision events (termed as twice grain-bed collisions). Analysis show that when the windblown sand concentration is higher than a critical concentration Q(cr)., the number of ejected sand grains given by twice grain-bed collisions is 23% higher than that by two-grain-bed collisions. Therefore, two-grain-bed collisions considering the interaction of two grains simultaneously impact a sand-bed is a significant factor that cannot be overlooked in numerical schemes of windblown sand flux. (C) 2012 Elsevier B.V. All rights reserved.