To reveal the coupling mechanisms of reinforcement ratio, steel fiber dosage, and segment thickness on the flexural performance of steel reinforced steel fiber concrete (R-SFRC) shield segments, this study combined experimental testing and refined numerical simulation. The load-deflection responses and damage evolution of curved specimens with different fiber dosages were obtained through four-point bending tests. A three-dimensional meso-scale finite element model, incorporating the random distribution of steel fibers and interfacial behavior, was subsequently developed and validated against experimental data. Using this validated model, a systematic analysis was conducted on three key parameters: reinforcement ratio (0.30%-0.84%), steel fiber dosage (0-30 kg/m3), and thickness (250-300 mm). The results demonstrate a dynamic stress redistribution synergy between rebar and fibers, with fiber bridging and multi-cracking control being more pronounced at lower reinforcement ratios. A saturation threshold for the steel fiber enhancement effect was identified at approximately 20 kg/m3, beyond which the enhancement of stiffness becomes minimal. Segment thickness is the most sensitive parameter, as the “weakening effect” from section reduction significantly increases rebar stress and cannot be fully offset by fiber reinforcement alone. The research results can provide theoretical basis and parameter optimization methods for the refined and collaborative design of high-performance shield tunnel segments.