Abstract: To enhance the durability of mine backfill in complex geological environments, this paper investigates the performance modification effects and deterioration mechanisms of polypropylene (PP) fibers and rubber particles on backfill subjected to the coupled action of sulfate attack and dry-wet cycles. Dry-wet cycle erosion tests (conducted under conditions of Na₂SO₄ concentration ≤ 10% and dry-wet cycles ≤ 60) and microscopic multi-scale characterizations including XRD, SEM, MIP, and TG/DTG were carried out to systematically study the macroscopic mechanical responses and microstructural evolution laws of the mixed materials. The results indicate that although the combination of fibers and rubber slightly reduces the fluidity of the slurry and prolongs the setting time, the mixed specimens with 5% rubber content and 0.3% fiber content exhibit a UCS increase of 4.1% compared to the reference group, reaching 5.57 MPa, and an STS increase of 34.6%, reaching 1.44 MPa, while demonstrating good erosion resistance. Microscopic analysis reveals that the expansive crystals generated by sulfate attack lead to matrix cracking and gel decomposition. PP fibers effectively inhibit crack propagation and reduce pore connectivity, while rubber particles absorb expansion stress through interfacial elasticity and hinder ion transport. The synergistic effect of the two maximally delays the structural damage process of the material. This study reveals the durability enhancement mechanism of multiphase composite reinforced backfill, providing a theoretical basis and technical reference for mine backfill engineering affected by sulfate attack.