Abstract: Granular materials typically exhibit pronounced anisotropic behavior under shear conditions. Investigating the evolution of anisotropy is essential for gaining deeper insight into the macroscopic mechanical behavior of granular materials. In this study, undrained shear tests were conducted on granular materials using a strain-controlled photoelastic testing apparatus to investigate the mesoscopic topological evolution of shear-induced anisotropy of the granular assembly. The results show that fabric anisotropy of the granular assembly develops rapidly in the early loading stages and tends to be stable upon reaching a steady state. The experimental results further validate the effectiveness of the stress-force-fabric relationship. By categorizing the granular system into strong contact system and weak contact system, it was observed that the strong contact system exhibits much higher anisotropy than the weak contact system. This suggests that the anisotropy of the overall system is primarily governed by the strong contact system. The anisotropy and force transmission characteristics of force loops of different sizes were also examined. The results show that the contribution of force loops of different sizes to overall anisotropy varies significantly. Specifically speaking, force loops with larger sizes tend to exhibit higher anisotropy, whereas smaller force loops, especially L3, are more stable structures. Force loops were further categorized into strong force loop system and weak force loop system based on the magnitude of their principal stresses. Among them, L3 force loops have the highest participation in the strong force loop system, followed by L4 and L5, while the L5 + loops contribute the least. The results highlight the crucial role of small force loops in resisting deformation and carrying external loads, thereby ensuring the stability and integrity of the granular system under shear conditions. Finally, the influence of particle size distribution on anisotropy was analyzed, revealing that specimens with a wider gradation exhibit more pronounced anisotropic characteristics.