Abstract: Traditional backfill relies on cement as a binding cementing, which is prone to brittle failure under complex geological conditions, leading to backfill failure and posing a threat to underground safety. Therefore, this paper investigates steel fiber and polypropylene fiber as reinforcing materials, focusing on steel fiber-reinforced backfill, polypropylene fiber-reinforced backfill, and mixed fiber-reinforced backfill. Through indoor mechanical tests, DIC full-field strain monitoring, SEM microscopic tests, CT scanning technology, and PFC3D numerical simulations, the influence mechanism of fiber type and fiber content on the early mechanical behavior and failure mode of backfill was system atically revealed. The results show that steel fiber and polypropylene fiber can improve the uniaxial compressive strength and splitting tensile strength of the backfill, with polypropylene fiber having a more significant effect on tensile strength. Both single fiber and mixed fiber systems have an optimal content. After exceeding the optimal content, the mechanical properties decrease to varying degrees, but they remain higher than those of the fiber-free backfill specimens. The DIC full-field strain test shows that the fibers delay crack propagation through the bridging effect, enhance the strain distribution uniformity of the mixed fiber specimen, and improve its deformation resistance compared to the single fiber backfill. Numerical simulations show that the incorporation of fibers significantly reduces the number of tensile cracks, with the crack resistance effect following the order: mixed fiber > polypropylene fiber > steel fiber. The mixed fiber specimen exhibits the best particle displacement inhibition effect. The microstructure analysis shows that steel fiber restrains crack propagation through interface friction, polypropylene fiber inhibits crack propagation through frictional energy dissipation, and the mixed fiber forms a three-dimensional skeleton that optimizes the load transfer path. The results of CT scanning indicate that an appropriate amount of polypropylene fiber makes the internal structure of the backfill denser, effectively blocking the initiation and evolution of microcracks. The inhibition effect improves the material's damage mode, ultimately leading to the optimization and enhancement of its macroscopic mechanical properties. The research results demonstrate that fiber hybrid incorporation improves the material's damage mode through a multi-scale synergistic mechanism, providing a foundation for the technical promotion of fiber-reinforced tailings-cemented backfill in mine filling engineering.