<p>This study investigates the time‑independent and time‑dependent mechanical behavior and fracture evolution of fractured crystalline sandstone under various loading conditions. Using the Voronoi polygon tessellation method, we established a discrete element numerical model that accounts for the heterogeneity of sandstone containing pre‑existing flaws. Subcritical crack growth theory was then incorporated, and a heterogeneous numerical model for time‑dependent deformation was developed through embedded FISH language (a scripting language originally developed for the Fast Lagrangian Analysis of Continua (FLAC) and implemented in UDEC). The mesoscopic parameters of the model were calibrated by adjusting them until the numerical simulations reproduced the macroscopic responses (e.g., uniaxial compressive strength and elastic modulus) obtained from laboratory tests on intact sandstone. Using this calibrated model, we then analyzed the time-independent and time-dependent deformation characteristics of sandstone specimens containing pre-existing flaws. The results demonstrate that as the pre-existing flaw angle increases, the mechanical properties progressively degrade, the failure mode shifts from tensile splitting to shear failure, and the failure time decreases. A longer pre-existing flaw intensifies stress concentration, further degrading the mechanical properties and accelerating time-dependent failure. Elevated confining pressure suppresses microcrack propagation and slows damage accumulation, substantially enhancing the mechanical performance of the specimens. From a mesoscopic perspective, this study reveals the mechanisms by which fracture geometry and confining pressure control time-independent and time-dependent deformation and failure in rock. These findings provide a basis for assessing the long-term stability and support design of fractured rock masses.</p>