AbstractAfter the Ms 8.1 Kunlun earthquake, postseismic displacements were resolved with GPS. The observed data showed that there were significant differences between the deformations on the south and the north sides of the Kunlun fault. In addition, it is shown that the direction of deformation on the north side shifted from westward to eastward after a short‐time adjustment. Such GPS data are used as the constraints in finite element method (FEM) modeling of postseismic deformation. Using 3D poro‐viscoelastic model, we show that the horizontal inhomogeneous viscosities in lower crust can play an important role in the geodynamic behavior. The best‐fitting viscosities of lower crust are about 5.0×1017 Pa s, 9.0×1018 Pa s in Qiangtang block and Qaidam basin, respectively. The simulation suggests that the postseismic deformation of 2001 Kunlun Ms8.1 earthquake probably be caused by both the viscoelastic relaxation and poroelastic rebound. Copyright © 2009 John Wiley & Sons, Ltd.