Using the standard k-ε turbulent model, the flow field over a dune was simulated numerically by use of the open source computational fluid dynamics software OpenFOAM. The velocity distribution curves for the flow field at different heights over the dune were obtained, the influences of the dune height and width on the characteristics of recirculation zone at the leeward side of the dune were explored and the cause of the formation of recirculation zone was analyzed. The results show that when the dune height is small, the velocity of the air close to the dune surface first decreases and then increases at the leeward side. As the dune height increases, the air velocity at the leeward side decreases more rapidly and becomes unstable, as a result, the recirculation zone appears. It is also found that the scale of the recirculation zone decreases with the increase of the aspect ratio of the dune when either the dune width or the dune height keeps constant. The size of the recirculation zone depends not only on the aspect ratio but also on the height and width of the dune. When the recirculation zone at the leeward side initiates, a series of small eddies appear close to the dune surface. With decreasing in the aspect ratio of the dune, both the size and the intensity of the eddies increase, generating the recirculation zone.