Desiccation cracks, serving as preferential pathways for both water evaporation and infiltration, are critical to the hydrological processes governing the stability of expansive clay slopes. The opening and closing process of desiccation cracks during drying and wetting periods strongly influence the preferential flow and soil strength. However, the effect of these dynamic changes of desiccation cracks on slope hydrology and stability remains unclear. In this study, we first conducted a non-failure slope model test under multiple drying-wetting cycles to investigate the interplay between crack dynamics and slope hydrology. Then, a new dynamic dual-permeability preferential flow model (DDPM) was proposed and validated via the experimental hydrological data. Building on the DDPM, we developed a hydro-mechanical slope model considering the weather-driven crack dynamics and their deterioration effect on soil strength. Comparative numerical experiments were conducted under one-year atmospheric conditions to investigate the groundwater level, water balance, pore water distribution, crack evolution and slope stability in the case of dynamic cracks and fixed cracks. The experimental findings revealed that weather-driven crack dynamics have a dual effect on slope water balance. The DDPM better described the underlying physics involving crack evolution and slope hydrology with respect to the single-domain model and rigid DPM. The hydrological modelling results from DDPM-based hydro-mechanical showed that the slope model with dynamic cracks retained more water and higher groundwater level than that with fixed cracks. The narrowing of desiccation cracks slows down slope drainage process, resulting in a rapid build-up of pore water pressure due to preferential flow, which emerges as a newfound factor contributing to slope instability. Conversely, fixed and well-connected cracks in soils enhance water drainage and thus benefit slope stability. The mechanical results revealed that the irreversible deterioration effect induced by crack dynamics on soil strength persistently degrades long-term slope stability. Our result underscores the importance of incorporating soil structure dynamics into slope hydrological analysis and highlights the consequence of weather-driven deterioration effect on slope stability.