Abstract: In order to explore the influence of steel fiber (SF) on the mechanical properties and damage and failure mechanism of backfill, fiber reinforced cemented tailing backfill (FR-CTB) was taken as the research object to study the influence of SF content on the Mechanical properties of backfill. Digital image correlation (DIC) technology was used to monitor the full-field strain of specimens and track the crack development of specimens. In addition, the strengthening mechanism of SF on backfill was further studied from the microscopic level. The results show that the uniaxial compressive strength, splitting tensile strength and shear strength of FR-CTB increase in varying degrees with the increase of SF content and curing age. The enhancement effect is the best when the SF content is 20kg/m3, but the enhancement effect decreases significantly when the SF content exceeds 20kg/m3. The existence of steel fiber restrains the crack expansion of the backfill to a large extent, weakens the stress concentration at the crack tip, effectively prevents the crack expansion, and improves the deformation of the whole specimen. Furthermore, the enhancement effect of SF is mainly reflected in bridge and pull out behavior. The hydration products increase the roughness of SF surface, increasing the friction between SF and cement-tailings matrix consequently to absorb the energy of external load and improve the mechanical properties of FR-CTB. Finally, SPSS curve estimation is used to establish the strength calculation model of each age backfill, which has high accuracy and can predict the strength of FR-CTB.