[Objective] To address the lack of a unified quantitative chain linking “microstructure-damage characteristics-macroscopic fragmentation behavior” in weathered granite, this study aims to reveal its mechanical degradation and damage evolution mechanisms under cyclic impact. A dual-scale fractal characterization framework connecting microscopic damage to macroscopic failure is constructed. [Method] Unweathered, moderately weathered, and strongly weathered granite samples from the Meijiang River Irrigation District tunnel project were tested. A dual-scale fractal analysis framework was proposed: micro-crack networks were extracted from SEM images to calculate the micro-crack fractal dimension (D_f) using the 2D box-counting method. Cyclic impact tests were conducted using a Split Hopkinson Pressure Bar (SHPB) under constant air pressure. Based on dynamic stress-strain responses, a weathering damage variable (D_W) coupled with impact cumulative damage was defined. Additionally, the macroscopic debris fractal dimension (D_S) was calculated from post-impact particle size distributions to quantify fragmentation characteristics. [Results] (1) As weathering progressed from unweathered to strongly weathered, mineral decomposition and secondary clay content increased, significantly enhancing micro-crack network complexity. The fractal dimension D_f increased from 1.72 to 1.95, indicating that weathering provides abundant initial defects for subsequent damage evolution. (2) Under fixed impact air pressure loading, specimens with higher weathering degrees exhibited faster peak stress attenuation (the quadratic coefficient decreased from -0.06 to -1.74), increased residual strain, and weakened strain rebound effects. The dynamic elastic modulus deteriorated with increasing impact cycles, with the deterioration amplitude of strongly weathered samples exceeding 50%, reflecting the significant weakening of dynamic fatigue resistance. (3) Weathering damage (D_W) and cumulative damage (D) exhibited coupled nonlinear growth. Unweathered and moderately weathered rocks showed a three-stage evolution of “compaction-stability-sharp increase”, whereas strongly weathered samples almost directly entered the sharp damage increase stage due to extensive secondary cracks. (4) Clear quantitative correlations existed between fractal parameters and damage variables. Both D_f and D_S increased significantly and linearly with increasing D_W (R²>0.90), achieving cross-scale linkage from microscopic damage characterization to macroscopic failure evaluation. [Conclusion] This study establishes a dual-scale fractal characterization framework linking microstructure to macroscopic failure in weathered granite under cyclic impact. The micro-crack fractal dimension (D_f) and debris fractal dimension (D_S) both increase linearly with the weathering damage variable D_W (R²>0.90), confirming that fractal metrics can effectively bridge microscopic damage accumulation and macroscopic fragmentation behavior. Weathering significantly degrades the dynamic fatigue resistance of granite: strongly weathered samples exhibit accelerated peak stress attenuation, elastic modulus deterioration exceeding 50%, and a direct transition to the sharp damage increase stage due to abundant initial defects. These findings provide a quantitative basis for assessing the stability of weathered granite in tunnel engineering under dynamic disturbance and offer a cross-scale evaluation method for rock damage characterization.