Maintaining the solubility of micellar casein (MC) powder during storage is a major practical challenge. This study investigated the effects of storage temperature (−20 to 37 °C) on solubility and structure of MC powder. Results showed that solubility of MC powder was well preserved at 4 °C and −20 °C over 30 d, whereas higher storage temperatures led to a marked decline. Correspondingly, particle size and stability of rehydrated solution from powders stored at lower temperatures remained unchanged but increased significantly at elevated temperatures, alongside visible precipitation. Structural analysis indicated that surface cross-linking and reduced porosity occurred during storage, resulting in decreased solubility, which were effectively suppressed at 4 °C and −20 °C. These findings demonstrate that refrigerated storage at 4 °C is sufficient to maintain the solubility of MC powder, with no clear additional advantage observed at −20 °C under the tested conditions. This work provides practical guidance for optimizing MC powder storage.