The mechanism by which ice slurry crystallizes under a vacuum was investigated in this work. The crystallization kinetic models for two different aqueous solutions (glycerol; a combination of glycerol and nano-SiO2) were developed based on the classic Avrami theory. The values of the Avrami exponent n under different experimental conditions were between 0.58 and 0.84, indicating the one-dimensional flake thickening growth of ice crystals. Glycerol inhibited the growth of ice crystals due to the hydroxyl-containing structure, and nano-SiO2, which served as a nucleating agent, could enhance the crystallization process effectively. The growth morphology of ice crystals, including the size and shape, were also observed. Results showed that ice crystals produced by the vacuum method were generally oval in shape. Adding nano-SiO2 to the glycerol solution reduced the size of the ice crystals, suggesting that nano-SiO2 is a promising additive for ice slurry systems. The study of crystallization mechanism of ice slurry can be useful for future design of vacuum ice production systems.