As a typical energy dissipation system, granular material acts as a buffer under the action of impact load, with absorbing and dissipating energy effectively through the sliding friction and viscous contacts between particles. In this paper we study the buffer capacity of granular material under impact load, by the discrete element method (DEM). The spherical elements are filled randomly into a rigid cylinder under the action of gravity. A spherical projectile with a certain initial velocity drops into the granular bed from a given height. The impact loads on the projectile and the rigid bottom plate of cylinder are both obtained with DEM simulations. The simulated impact loads on the bottom plate are compared well with the physical experiment data. The influences of granular thickness, sliding friction and initial concentration on buffer capacity are investigated under the impact of spherical projectile. The DEM results show that granular thickness H is a key factor for buffer capacity. In the DEM simulations, the impact load on bottom plate presents unique characteristics under various granular thickness values. With granular thickness increasing from zero, a transition from one peak to two peaks takes place, then the two peaks return to one peak in the time curve of impact load. The evolution of impact load peak with its temporal interval is discussed. A critical thickness Hc is obtained. The impact force decreases with the increase of granular thickness when H Hc, but is independent of the granular thickness when H Hc. Moreover, the impact forces are simulated under various sliding friction coefficients and initial concentrations. It is found that the smooth and loose granular material has more effective buffer capacity. Finally, the spatial structures of force chains and the distribution of impact forces on bottom plate are discussed to reveal the mechanism of buffer properties of granular material on a micro scale.