Abstract: Fiber reinforcement can mitigate the brittle failure of soil stabilized via enzyme-induced carbonate precipitation (EICP), demonstrating promising application prospects in environmental geotechnical engineering. To thoroughly investigate the synergistic toughening mechanism of the two materials, untreated soil, bamboo fiber (BF)-reinforced soil, EICP-stabilized soil, and EICP-BF stabilized soil were selected as research subjects. The effects of BF content and length on the toughness of modified soil were systematically examined through macroscopic mechanical tests, and the underlying mechanisms were elucidated via microstructural analysis. The results show that the unconfined compressive strength of EICP-BF stabilized soil first rises and then declines with increasing BF content and length, reaching a peak value of 1012 kPa at 2% BF content and 10 mm fiber length. The incorporation of BF exerts a significant influence on the post-peak brittleness index of EICP-BF stabilized soil, with the toughening effect reflected by a gentler post-peak drop in the stress-strain curve and an improvement in residual strength. The grooved surface of BF facilitates the precipitation and adhesion of calcium carbonate. The bridging interaction between BFs and calcium carbonate crystals, together with the formed BF skeleton network, jointly enhances the strength and toughness of the modified soil.