<p>This study investigates the damage and parameter evolution of sandy slate through graded cyclic loading and unloading tests under various stress paths. Based on an energy mechanism, a nonlinear evolution model for rock strength parameters (cohesion and internal friction angle) and dilatancy angle was established, using plastic shear stress as the plastic parameter. The model was subsequently implemented in numerical simulation software. The findings reveal that the damage variable, derived from the breakage energy (<i>U</i>_<i>b</i>) in dissipated energy, exhibits four stages: initial damage, accelerated damage, rapid damage, and decelerated damage. The rate of damage growth before failure follows the sequence: loading axial stress and unloading lateral stress (LAUL) &gt; unloading axial and lateral stress (UAUL) &gt; loading axial stress and constant lateral stress (LACL), indicating that the unloading process inflicts more severe damage on the rock’s internal structure. The evolution models for cohesion (<i>c</i>), internal friction angle (<i>φ</i>), and dilatancy angle (<i>Ψ</i>), based on plastic shear strain as an internal variable plastic parameter, accurately describe the nonlinear evolution characteristics of these parameters from plastic deformation to the residual stage, with <i>c</i> gradually decreasing and <i>φ</i>, <i>Ψ</i> initially increasing and then decreasing under different stress paths. By incorporating the parameter evolution model into the strain softening/hardening Mohr-Coulomb model in FLAC^3D for simulation calculations of tests, it effectively reflects the elastic phase, pre-peak strain hardening, post-peak softening, and the residual stage of rocks under different stress paths. The obtained distribution of the plastic zone corresponds to the failure modes observed in the experiments.</p>