Konjac glucomannan (KGM) with different degrees of deacetylation (DD) was prepared to examine the effects of DD on ice crystal growth and its potential as a cryoprotectant in frozen foods. Deacetylated KGM (DK) samples were characterized by FT-IR and AFM, and their effects on water activity and ice crystal size were assessed. Time-resolved near-infrared (NIR) spectroscopy at -4 °C, with wavelet packet transform (WPT), was employed to resolve ice- and water-structure-related bands during ice growth. Native KGM showed linear chains, whereas increasing DD led to more curled and aggregated conformations and lowered water activity (aw). DK with a DD of 34.3% (DK34.3%) exhibited the strongest inhibition of ice growth, producing the smallest ice crystals (11.0 ± 0.4 μm at 0.8%, w/w), while further deacetylation (e.g., DK65.6%) weakened the inhibition, likely due to enhanced aggregation and reduced interfacial accessibility. NIR analysis showed that the ice Ih (hexagonal ice) marker band at 6118 cm-1 increased most slowly for DK34.3%, indicating a prolonged induction stage and the lowest apparent ice growth rate. High-resolution spectral components further showed the emergence of a strongly hydrogen-bonded, ice-associated water feature near ∼6693 cm-1 and the attenuation of a hydration-related band around ∼6880 cm-1, consistent with water-structure reorganization near the ice-solution interface during ice growth. In a model ice-cream mixture, DK34.3% (0.8%, w/w) reduced the area-weighted mean ice crystal diameter to 5.6 ± 0.2 μm compared with the control (6.8 ± 0.3 μm). These findings indicate that moderate deacetylation improved cryoprotection by modulating aw, molecular conformation, and interfacial water behavior, providing an effective natural polysaccharide-based cryoprotective ingredient for frozen foods.