To study the influence of polyoxymethylene (POM) fibers on the mechanical properties of shotcrete for tunnel support, this research conducted four-point bending tests on concrete with different POM fiber dosages (0, 5, 7, and 9 kg/m³) and lengths (30 mm, 36 mm, and 42 mm). The mechanical properties are analyzed in terms of failure modes, flexural strength, and the toughness index. The results show that, with the increase fiber length and dosage, the incorporation of POM fibers can enhance the toughness of concrete and significantly improve the flexural performance of shotcrete, with the peak flexural strength increasing by 15.31% to 89.46%. Additionally, through scanning electron microscopy (SEM) image analysis, the reinforcing mechanism of POM fibers is revealed: when shotcrete with POM fibers is subjected to flexural loading, it undergoes four stages: elastic, elastic–plastic, yield, and failure. The addition of POM fibers increases the density and uniformity of concrete, and the flexural strength is indirectly enhanced by increasing frictional energy dissipation through the formation of fiber–matrix interfaces between fibers and concrete. The research findings provide a theoretical basis and design reference for the application of POM fiber-reinforced shotcrete in tunnel support.