Abstract: Rubber-sand mixtures exhibit potential for applications in foundation and vibration mitigation engineering due to their favorable deformation compatibility and energy dissipation capacity. However, the pronounced contrast in stiffness and deformability between rubber and sand particles renders the small-strain mechanical behavior and stress transfer mechanisms difficult to characterize using conventional state parameters. In this study, a discrete element method is employed, in which non-spherical sand particles are constructed using a multi-sphere assembly technique, to systematically investigate the influence of particle shape on stress distribution and small-strain shear stiffness of rubber-sand mixtures. A dual-scale stress decomposition framework is adopted to quantitatively assess the contributions of different particle types and contact types to stress transmission. The results show that the stress carried by rubber particles is generally lower than their volume fraction and further decreases with increasing confining pressure and decreasing density. Pronounced differences in contact stiffness among contact types lead to the failure of conventional state parameters in capturing small-strain shear stiffness. By treating rubber particles as equivalent pores under small-strain conditions, a modified state parameter is proposed, which enables a unified description of the small-strain shear stiffness.