Abstract: This study investigates the fragmentation expansion and water storage characteristics of collapsed rock in overlying strata during its formation. Pre-static loaded fractured samples of four typical roof rock types-coarse sandstone, fine sandstone, sandy mudstone, and mudstone-were tested for their water storage coefficients under 10-30 J of impact energy. Specifically, we analyzed their fragmentation expansion and water storage structures, along with their fractal dimensions. Results show that rock samples exhibited further expansion of their water storage spaces under the impact, forming a "skeleton-network" composite structure. Their fractal dimension ranged between 1.59-2.16, and their average block size between 30.19-35.19 mm. Under identical impact energy, the degree of fragmentation expansion and water storage coefficient decreased sequentially from mudstone, coarse sandstone, fine sandstone to sandy mudstone due to differences in their impact resistance. With rising impact energy, these four rock types showed significant increase in their fragmentation expansion and water storage coefficients after impact, It demonstrated a "gentle growth, rapid rise, gentle growth" pattern, as was described by a logistic model. There is a strong correlation between the water storage coefficient, fractal dimension, and average block size of rock samples under different impact disturbances, indicating that water storage coefficient is governed by the synergistic effect of fractal dimension and average block size. This study thus constructed a formula for predicting water storage coefficient based on fractal dimension and average block size, which offers new insights for predicting water storage space of underground reservoirs.