Abstract: Cemented tailings backfill (CTB) technology, an innovative mode of tailings management, has been widely applied in many metal mines worldwide due to its advantages of safety, environmental protection, and high economic benefit. During the mining process, CTB should have sufficient mechanical strength to maintain the stability of the underground stopes and provide a safe environment for workers and mining equipment. However, in deep mining, cracks and imperfection in CTB are usually generated by the extraction of adjacent stopes, blasting disturbances, and stress concentration. Existence of these cracks weakens the engineering properties. It causes instability of backfill stopes and increases ore dilution. At present, the mechanical strength of CTB structures is improved by increasing binder content, which directly leads to an increased backfilling cost. Hence, to solve the problems mentioned above, CTB specimens were prepared with cement-tailings ratios of 1∶10 and 1∶20, and polypropylene fiber contents of 0, 0.05%, 0.15%, and 0.25% (by dry weight of tailings and cement). The effect of fiber content on the mechanical strength and deformation properties were investigated by conducting unconfined compressive strength (UCS) tests. Referring to scanning electron microscopy (SEM), the mechanism of fiber reinforcement is discussed. Results indicate that the yield stress of fresh CTB mixtures increase linearly with increasing fiber content, and the rheological characteristic of the mixtures conformed to the behavior of Bingham. UCS values of CTB increase with increasing fiber content, but decrease when the fiber content is > 0.15%. Optimal fiber content is 0.15%. It is found that fibers can effectively delay the expansion of cracks and constrain the deformation of backfill. The post-peak strain softening and residual strength are improved by the addition of fibers. Failure characteristics of CTB are transformed from brittleness to ductility due to the mixed fibers. The reinforcement effect of fiber is mainly controlled by the adhesion and friction between fibers and tailings-cement matrix. The overall objectives are to improve current understanding of the mechanical properties of CTB, thereby reducing the risk of clogged pipelines and higher backfilling costs as well as improving the stability of CTB structures.