Uniaxial compression and scanning electron microscopy (SEM) analyses were conducted on cemented tailings backfill (CTB) specimens with varying fiber contents and lengths to investigate strength enhancement mechanisms. Results indicated a non-monotonic relationship between average uniaxial compressive strength (UCS) and both fiber content (optimal range: 12‰-18‰) and length. Optimal fiber content established hydration product-bridging skeletal frameworks to constrain crack propagation, while appropriately sized fibers enhanced plastic deformability through robust interfacial bonding without compromising load-bearing capacity. Fibers inhibited primary crack formation via bridging while providing nucleation sites for calcium silicate hydrate (C-S-H) and ettringite (Aft) to facilitate crack repair. During crack initiation, fibers anchored fracture surfaces, dissipated crack-tip energy, and restricted microcrack advancement.