ABSTRACT In order to investigate the dynamic shear characteristics of rubber sand under cyclic loading, a series of cyclic torsional shear tests was conducted using a hollow cylinder torsional shear apparatus. The effects of four cyclic vertical stress ratios (CVSRs = 0.15, 0.25, 0.35, and 0.45) and four cyclic torsional stress ratios ( η = 0, 1/6, 1/3, and 1/2) on the strength and volumetric deformation characteristics of rubber sand were analyzed. Based on the tests, a three‐dimensional undrained discrete element model of a hollow cylinder torsional shear test was developed to further examine the evolution of particle motion, porosity, coordination number, and fabric anisotropy during the shearing process. The results show that with increasing cyclic vertical and torsional stress ratios, horizontal reconstruction of shear bands becomes more pronounced, and axial strain accumulates more rapidly. When CVSR and η are relatively small, the shear modulus decreases gradually with their increase, and the damping ratio remains relatively stable. At the mesoscopic level, the contact network exhibits a decrease in coordination number, a rapid increase in porosity, and an accelerated decay of normal contact force. When CVSR and η are relatively large, the particle skeleton undergoes rapid liquefaction and reorganization, with the shear modulus and damping ratio decreasing sharply. At this stage, the number of normal contacts is reduced to a minimum, and the normal contact force almost completely disappears.