Abstract: The granular materials are widely used in geotechnical engineering, whose grading changing feature will affect their mechanical behavior obviously, especially the bearing capacity. In order to well describe the influence of grain-size distribution on the mechanical behavior of granular materials, a simple constitutive model taking into account the grading-dependent critical state line is developed within the framework of elasto-plasticity and the critical state theory. The model is used to simulate the drained and undrained triaxial compression tests on different types of granular materials (DEM ideal sphere, artificial material glass ball and natural material Hostun sand). It is found that only one group of grading-dependent critical state parameters is needed for describing the mechanical response of granular materials with different gradings.