Granular flow exists widely in the fields of natural environments and industrial production. Despite the fact that external driving forces are recognized to have a significant effect on the dynamic of granular systems, a large number of experimental measurements and numerical simulations have been contributed to the study of granular flow considering gravity as the sole driving force. In this work, the granular flow driven by external pressure is simulated by GPU-based DEM (discrete element method). To examine the validity of the DEM model, the corresponding experimental results for spherical granular materials and the traditional Beverloo equation are compared. Furthermore, the effects of gravity acceleration, orifice diameter, and particle friction on the flow rate under different external pressures are studied. Finally, the distribution of the particle velocity and the normal contact force between particles are analyzed to demonstrate the influence of external pressure on the macroscopic discharge rate. (C) 2019 Elsevier B.V. All rights reserved.