The Yinchuan graben in north central China is an intense active zone that is bordered by the Alashan to the west and the Ordos to the east. Analysis of high-resolution digital topography shows that the present elevation on two shoulders of the graben exists drastic contrast (~1.5--2 km). We use two-dimensional finite element modeling to investigate the mechanical relation between crustal rheology and flank uplift of this graben. Models are simplified as an elastoplastic upper crust and a viscoelastic lower crust. Two antithetic faults on the graben's margins are simulated as Coulomb-type frictional zones. Rheological tests show that the asymmetric flank uplift of the graben can be reconciled with lateral variations of crustal rheology; With the lower-crust viscosity of the Ordos being ~1--2 orders greater than that of the Alashan, we predict that even with the same effective friction of the two antithetic faults (0.1), the flank uplift will localize mainly on the west shoulder, consistent with present topographic contrast. This means that the crustal strength of the Alashan is weak relative to the Ordos, resulting in the active deformation and the seismicity being occurred heavily within the Alashan.