Traditional roadside backfill bodies made of high-water-content material (TRBBs) are commonly adopted in gob-side entry retaining (GER). However, their brittle response often results in crack propagation, loss of load-carrying capacity, and instability under roof rotation and mining-induced stress. This study investigates the fracture-resistance mechanism, damage evolution, and field performance of a fiber-reinforced roadside backfill body (FRBB). A fracture-mechanics-based model was developed to quantify the toughening effects of fiber bridging and fiber pull-out, and Mode I fracture toughness was measured using semi-circular bending tests. The optimum polypropylene fiber dosage was 0.3%, at which fracture toughness increased from 0.130 to 0.210 MPa·m0.5, a 61.5% enhancement relative to the plain matrix. A 3DEC model incorporating strain-softening behavior, staged material-parameter updating, and joint energy-based damage indicators was used to simulate the response of the surrounding rock and FRBB deterioration. The simulations show that stress concentration and damage mainly occur 30–80 m behind the working face, where fiber reinforcement retards crack interaction and improves damage tolerance. Parametric analyses indicate that greater GER width and longer key block B reduce stability, whereas a thicker immediate roof and larger FRBB width improve structural performance. The proposed analytic hierarchy process (AHP)–matter–element extension model showed that the GER stability grade improved from IV in the TRBB case to III in the FRBB case. Field monitoring showed roof-to-floor and rib-to-rib convergence values of 490 mm and 370 mm, respectively, with only slight FRBB surface microcracking. These findings provide a mechanical basis for designing GER systems with FRBB.