<p>This study investigates the mechanical behavior and microstructural evolution of yellow and gray sandstone under cyclic stress loading. An experimental program comprising uniaxial compression tests (UCT), triaxial compression tests (TCT), uniaxial cyclic stress tests (UCST), and triaxial cyclic stress tests (TCST) was conducted using a servo-controlled triaxial testing system integrated with digital imaging technology. Results show that cyclic loading enhances the compressive strength of sandstone specimens by up to 20%. This strengthening effect arises primarily from the densification of the granular structure and the closure of microcracks, which improve interparticle contact and expand the internal force chain network. During cyclic loading, the preferential formation of axial microcracks leads to a gradual decrease in Poisson’s ratio with increasing cycle numbers. The porosity ratio exhibits a similar trend in both uniaxial and triaxial tests: it decreases sharply at the onset of each stage, shows a brief rebound at the beginning of the second stage in uniaxial tests, and then stabilizes progressively. The evolution of Poisson’s ratio and porosity ratio suggests that microcrack generation and closure coexist during cyclic loading. Analysis based on the Drucker–Prager (D–P) failure criterion in <i>p</i>–<i>q</i> stress space reveals a parallel shift of the failure envelope, indicating increased cohesion with negligible change in the friction angle. Overall, the study demonstrates that cyclic stress triggers competing damage and strengthening processes, where initial compaction-induced strengthening transitions to damage accumulation with increasing stress amplitude. These findings offer critical insights into the evaluation and prediction of sandstone stability under cyclic loading conditions, particularly in tunnels and foundations.</p>