Abstract: Based upon shear strain gradient plastic theory of rock deformation localization, it is assumed that the failure pattern of rock in the shear band is shear failure. The confining pressure and the pore pressure are involved in the model and a theoretical stress strain relationship is obtained. The result of the theoretical analysis coincides with many experimental results. The localization is the cause of the confining pressure and pore pressure. The theoretical analysis has theoretical and practical significance for prevention from some natural hazards.