Buffer/backfill materials in high-level radioactive waste repositories are prone to desiccation cracking under thermal-hydro-mechanical coupling, which threatens the long-term isolation performance of the barrier system. To quantitatively investigate the influence of initial water content on crack evolution and barrier performance degradation, a buffer material composed of Gaomiaozi sodium bentonite mixed with 30% quartz sand was compacted to a dry density of 1.7 g·cm?³. Specimens with initial water contents of 10%, 15%, 20%, and 25% were subjected to drying tests at 70 °C. Crack geometric parameters were extracted via image processing, and a crack damage factor D was defined based on the cubic law. The results indicate that desiccation cracking of the compacted mixture follows a three-stage evolution pattern: initial cracking, crack development, and stabilization. Initial water content plays a decisive role in determining crack network morphology: specimens with low water content develop fine, polygonal crack networks originating from multiple initiation sites, whereas those with high water content form tree-like branching systems centered on wide primary fractures. As initial water content increases, the crack ratio, maximum width, and aspect ratio rise, while the total number of cracks decreases; the damage factor D exhibits exponential growth, revealing the high damage potential of “few but wide” crack networks under wetter conditions. Additionally, the rigid sand skeleton formed by 30% sand content effectively restricts vertical crack propagation, with a maximum crack depth not exceeding 3.4 mm. The innovation of this study lies in the construction of the crack damage factor D based on the cubic law, which overcomes the limitation of single geometric parameters and enables a comprehensive mapping from crack geometry to barrier functional degradation, providing a novel evaluation approach for quantitatively assessing the deterioration of buffer material sealing performance induced by desiccation cracking.