In order to reveal the change law of the strength characteristics, deviator stress-axial strain curve and volume strain-axial strain curve of the rubber sand mixture, the particle flow software is used to perform three-dimensional discrete element analysis on the mechanical properties of the rubber sand mixture under triaxial compression. Numerical simulation analyzes the influence of mass ratio, particle size ratio and confining pressure on the mechanical and deformation characteristics of rubber sand from both macroscopic and mesoscopic aspects. The analysis results show that when the rubber content is less than 10%, the rubber sand exhibits sand-like mechanical properties, that is, the first shear shrinkage and then the dilatancy, and the stress-strain curve is softened. When the rubber content is greater than or equal to 20%, the rubber sand exhibits rubber-like mechanical properties, specimens are monotonously shrunk, and the stress-strain curve is hardened. With the increase of rubber content, the peak value of the stress-strain curve of rubber sand decreases. The particle size ratio of mortar sand has a certain influence on the stress-strain relationship of rubber sand. The main manifestation is that as the average particle size ratio increases, the volume strain increases, the stress-strain curve decreases, and the shear strength decreases. As the confining pressure increases, the dilatancy and strain softening properties of low rubber content rubber sand are suppressed, and the shear shrinkage and strain hardening properties of high rubber content rubber sand are enhanced. The decrease of the shear strength of the mixture is mainly attributed to the loss of stiffness of the rubber sand sample caused by the addition of soft rubber particles, and the reason why the expansion reaction is inhibited is that the rubber particles are easily squeezed to produce volume shrinkage.