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. The results show that fabric anisotropy of the granular assembly develops rapidly in the early loading stages and stabilizes 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 anisotropy is primarily governed by 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. Specifically speaking, force loops with larger sizes exhibit higher anisotropy, whereas smaller force loops, especially L3, are more stable structures. Force loops were further categorized into strong 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, ensuring the stability and integrity of the granular system under shear. Finally, the influence of particle size distribution on anisotropy was analyzed, revealing that specimens with a wider gradation exhibit more pronounced anisotropic characteristics.