This study investigates the flexural behavior of UHPC materials containing steel fibers of varying lengths. Mesomechanical analysis is performed according to the analytical pullout model, using actual measured fiber distributions. The flexural behavior of UHPC after cracking is simulated by superposing the fiber bridging and matrix softening curves. Subsequently, the evolution of stress–strain along the axial crack of UHPC is evaluated under different loading conditions. The obtained results indicate that longer steel fibers can efficiently improve the flexural behavior of UHPC. Moreover, the actual fiber orientation and pullout length distributions can be well fitted by two-parameter probability distribution (PDF) and Gaussian functions, respectively. Based on the micromechanical analysis, the fiber bridging behavior is overestimated under the assumption of 2D random fiber distribution, and it is underestimated under the assumption of 3D random fiber distribution. Interestingly, the length of the shear lag zone is independent of the fiber length, which is about 20.16 mm. The obtained results demonstrate that the compressive stress and strain at the top of the tested specimen gradually transform into tensile stress and strain at the opposite side. Interestingly, the stress of the specimens with fiber length of 20 mm are slightly higher than the specimens with fiber lengths of 6 and 13 mm, and the strain just lower than the specimens with fiber lengths of 6 and 13 mm in all crack propagation stages, respectively. This indicates that, in addition to the number of steel fibers, the stress/strain dispersion depends on fiber length.