The dawn-dusk asymmetry of the magnetosheath under quasi-steady states has been studied by using a newly developed 3D MHD magnetosphere simulation model. The results show that the dawn-dusk asymmetry is substantial because of the Parker spiral IMF. It is found that the dawn-dusk magnetosheath thickness asymmetry is the effect of different shock conditions. The plasma density and flux asymmetry are mainly caused by the different thickness of the dawn-dusk magnetosheath, and the magnetic reconnection on the magnetopause has no significant effects. It is also showed that the Plasma Depletion Layer in front of the dayside magnetopause can cause duskward plasma flow, and the total plasma flux on the dusk side will be higher.