The strain localization of a water-saturated porous cohesive geological material is studied based on numerical solutions using a strain-gradient dependent viscoplastic constitutive model. Second order gradient of Volumetric viscoplastic strain is introduced into constitutive model to account for the non-local effect of the motion of microstructure. The viscoplastic strain gradient dependent model is numerically studied by a finite element method (FEM) using a quadratic interpolation function for viscoplastic strain. In the FEM analysis, the Blot's two-phase mixture formulation and the updated Lagrangian scheme are used for water saturated material. The width of the shear band and the mesh size sensitivity in finite element calculations are discussed based on the numerical results.