Abstract: Within the multi-mechanism framework, a novel constitutive model for sands is proposed based on the critical state and bounding surface plasticity theories. The model assumes that the macroscopic deformation behavior of sands can be obtained by summation of the contributions from a macroscopic volumetric mechanism and a set of virtual one-dimensional microscopic shear mechanisms with random orientations in space. Each microscopic shear mechanism describes a shear deformation and a volumetric deformation due to dilatancy, which are modeled by both the microscopic shear stress-strain relationship based on the macroscopic bounding surface plasticity theory and the microscopic stress-dilatancy relationship, respectively. Both the strength criterion and the stress-dilatancy relationship introduce a state parameter for compatibility with the critical state theory. The correlations between some microscopic and macroscopic model parameters are formulated for the triaxial compression under constant confining stress. The model contains thirteen parameters and most of them are defined by soil parameters with the clear physical meanings. The systematic comparisons between the model simulations and the test data indicate that the proposed model has an excellent capability in predicting sand responses under the drained and undrained monotonic loadings, and the rotation of the principal stress axes without using additional parameters.