Owing to the special geological background of Taiwan, weak rock in western Taiwan sedimentation strata has shear dilation and creep deformational behaviors. During the underground excavation, often have a large amount of compression deformation then result in disaster. General deformed characteristics for geo-materials are nonlinear. Using linear elastic model to compute the deformation of geo-materials will always misestimate. So it is necessary to develop a constitutive model that apply to geo-material. This research, based on the results of tri-axial test with the sandstone in western Taiwan that performed by Rock Mech. Lab. of NTU. To sum up the following deformation behavior of sandstone: (1) under shear loading will transform from its original isotropy to a shear-induced anisotropic (2) the shear dilation behavior will still exist under creep test According to the deformation behavior of sandstone, the concepts of shear modulus softening and anisotropic factor are used to develop an anisotropic softening model. And combine with the visco-elastic model to develop a time dependent model for anisotropic softening of sandstone. Then write subroutine of the model to apply it to numerical analysis. The model with three parameters bulk modulus K, shear modulus G and anisotropic factor β to simulate instant deformation and another three parameters Kc, Gc and viscosity η to simulate creep deformation. With several sets of pure shear tests and creep test we can get all the parameters. Through the simulations of different confined pressure and different stress path tests, it is found that good agreements with instant deformation of sandstone, but the creep deformation in high shear stress state are have some different. However the trend of simulations are consistent with the experimental results. Apply the model to finite element method of ABAQUS program. Writing user subroutine to define the material's mechanical behavior. Before analyzing the tunnel cases, single element test was applied to verify the accuracy of the subroutine. Then apply the model to tunnel excavation analysis. Comparing prediction results between the anisotropic softening model, elastic model and Drucker-Prager model. The numerical results show that, with the proposed model there is a uniform compression zone around tunnel section and the inward displacements (including the crown, sidewalls and invert) based on anisotropic softening model are larger then elastic model and Drucker-Prager model. Keywords: sandstone, constitutive model, softening, shear dilation, creep.