Abstract: Key to stability of gob-side entry retaining lies in stable bearing capacity of roadside backfill body (roadside support body). Traditional roadside backfill bodies exhibit inadequate adaptability to the roof structure in terms of mechanical properties such as strength and toughness, making them prone to brittle failure and the formation of through-going gas-conducting channel, thereby affecting safe mining at working face. This study proposes a method for mechanical modification of roadside backfill materials using fibers. A mechanical model for crack initiation and propagation in fiber-modified backfill body was established, elucidating the intrinsic mechanism by which fibers enhance material strength and toughness by improving the stress field at matrix crack tip. Experimental tests determined the influence characteristic of fiber content, length, and type on the mechanical properties of the fiber-modified high-water materials backfill body, and polypropylene fibers with a length of 6 mm and a dosage of 0.3% were selected as the optimal modification parameters, increasing the peak strength of the fiber-modified high-water materials backfill body by 21% while significantly enhancing its toughness and deformation capacity. Meso-mechanical simulations show that fibers exert a bridging effect within the fiber-modified high-water materials backfill body, effectively maintaining the mechanical connection between particles, suppressing the total amount and propagation rate of cracks, improving the spatial distribution morphology of cracks, and enhancing the uniformity, integrity, and stress transfer efficiency of the force chain network. Using the 150110 tailgate in Ping’an Coal Mine as a case study, a mechanical interaction model between the roadside backfill body and the roof was established considering the roof fracture and stability characteristics of gob-side entry retaining. It was clarified that the roadside backfill body should possess sufficient support strength (support resistance not less than11.5 MPa) and deformation capacity (peak strain reaching 6.5%) to accommodate roof rotation and subsidence (deformation amount reaching 195.0 mm). A discrete element numerical model of the gob-side entry retaining surrounding rock was constructed, and a time-dependent simulation method for the mechanical properties of the fiber-modified roadside backfill body was developed. A comparative analysis was conducted on the roof caving characteristics, crack propagation and damage degree of the roadside backfill body, and mining-induced stress evolution under the unmodified and fiber-modified backfill body conditions. The simulation results show that high-strength and high-toughness properties of fiber-modified backfill body significantly shorten rotation process of key block B, reduce its rotation subsidence, optimize the surrounding rock stress distribution, and reduce the tensile and total damage of roadside backfill body by 27.1%～30.8% and 19.5%～22.3%, respectively. Based on these findings, a synergistic control technology for gob-side entry retaining integrating high-strength and high-toughness roadside backfill body, roof bearing reinforcement, and zonal and time-phased support was proposed and successfully applied in 150110 working face of Ping’an Coal Mine. Field monitoring demonstrated that after adopting the fiber-modified backfill body for gob-side entry retaining, the roof-to-floor and rib-to-rib convergences are reduced by 135 mm (a reduction of 21.8%) and 96 mm (a reduction of 15.9%), respectively, significantly improving surrounding rock stability and meeting the requirements for safe mining in the subsequent panel. This research provides a novel approach for the design and performance optimization of roadside backfill materials for gob-side entry retaining and holds significant theoretical significance and engineering value for advancing non-pillar coal mining technology.