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Tailings were used to prepare cemented paste backfill (CPB) reinforced with polypropylene (PP) fiber, and fiber content and fiber length were 0-20 % and 3-12 mm, respectively. The Brazilian indirect tensile strength tests, macrostructural and microstructural (SEM) failure mode analyses and nuclear magnetic resonance (NMR) tests were performed. The results demonstrate that PP fiber with high tensile strength can substantially enhance the tensile strength and ductility of CPB specimens. The fiber-reinforced CPB exhibits superior performance during the pre-peak and post-peak stages and can retain its residual strength after being broken. The fiber-reinforced CPB provided a higher secant Young's modulus and strain at peak stress than that of the unreinforced CPB. From the failure mode analysis, fiber was used to bridge the CPB microelements and combine these effectively with calcium silicate hydrate gelling. The governing failure mode of fiber is pulled out or pull off, through which fiber absorbs the tensile stress and energy. However, fiber is also believed to have confusion distribution in CPB because fiber cannot reach their full strain capacity, especially at the later curing stage. And the weak structural layers in the CPB can also be formed, and the higher tensile strength is not achieved with the additional fiber content or longer fiber length. The optimal fiber content in this study is recommended to be 0.15 %, and it's better to choose 6 or 9 mm fiber. Moreover, fiber can reduce the original porosity by 10.72 %, but only has a slight influence on the pore size. The decrease in porosity helps increase the tensile strength of the fiber-reinforced CPB.