Abstract: In order to improve the toughness and mechanical properties of cemented tailings backfill in deep mines, the effects of temperature and fiber content on the macroscopic mechanical behavior and microstructure of the backfill were systematically investigated. Cylindrical samples with a lime sand ratio of 1∶6 and a slurry concentration of 70% were prepared and doped with 0~0.5% glass fiber respectively. Uniaxial compression tests, low-field NMR and scanning electron microscope analysis were carried out after curing at 25~50 ℃ for 28 days. The results showed that at a fiber dosage of 0.3% and a curing temperature of 45 ℃, the backfill’s compressive strength reached 7.578 MPa, which was 39.76% higher than that of the sample without fibers. The toughness index was 44.8% higher than that of the sample without fibers. This improvement was attributed to the high temperature accelerating the hydration reaction, forming a compact C-S-H gel with a more compact internal structure, thereby increasing strength gradually. Microscopic analysis showed that fibers effectively bridged cracks and filled pores, but excessive fibers resulted in agglomeration defects. NMR results further indicated that appropriate temperature and fiber content optimized the pore size distribution and reduced porosity. A damage constitutive model based on the Weibull distribution verified the synergistic effects of temperature and fiber content on the damage evolution of the backfill, providing a theoretical and experimental basis for the mining design of backfill in deep mines.