The mechanical behaviors and properties of rubber-sand mixtures are interesting and of practical importance but are not fully understood yet. In this study, 168 direct shear tests were performed to investigate the shear behaviors and strength of rubber-sand mixtures by considering various rubber content and rubber particle sizes, with an analysis made from the energetic and micromechanical perspectives to illuminate the underlying mechanisms. The experimental results indicate that increasing the rubber content helps reduce the shear strength of rubber-sand mixtures and restrain the mobilization of dilatancy, whereas it aids in promoting the ductility. The role of rubber particle size relies on the rubber content level: increasing the rubber particle size basically weakens the shear strength and dilatancy of rubber-sand mixtures with a relatively high rubber content, but it contributes to a minor increase in the shear strength at large shear displacements, given a low rubber content. The stress-dilatancy behaviors of rubber-sand mixtures can be characterized by a general linear function, in which the dilatancy coefficient ξ and critical state friction angle ϕ_cs, as two crucial function parameters, are both variables affected by the rubber content, rubber particle size and normal pressure. The energetic and micromechanical analysis based on the idealized conceptual model reveals that the impact of rubber content, rubber particle size, and normal pressure on the critical state shear strength is fundamentally linked with a combined effect stemming from the roughness of shear plane and relative number of particle sliding and rolling behaviors, the variation of which is in essence governed by the deformation of rubber particles.