The magnesium borate system’s dynamic behavior was investigated using ATR-FTIR and a self-made continuous humidity adjustable device. The supersaturation transformation and crystallization process of the three kinds of Ascharite, including Hungchaoite, Mcallisterite, and Inderite, were studied. Continuous adjustment of the relative humidity of the environment in the pool was achieved, allowing continuous monitoring of the dynamic chemical behavior of the magnesium borate supersaturation during evaporation. The supersaturation liquid film prepared on the substrate surface gradually evaporated and condensed into an ultra-concentrated state, eventually forming a multiphase mixture containing various magnesium borate crystals. Continuous measurement of infrared spectra of magnesium borate supersaturation on the substrate surface and magnesium borate crystals obtained by phase transformation and crystallization, combined with density functional theory calculations of infrared vibration patterns of various magnesium borate crystals, the supersaturation of poly borate in the three magnesium borates, the distribution of poly borate ions during the phase transformation, and the types of magnesium borate crystals formed by the final crystallization were described, the difference of species distribution of borates in three kinds of magnesium borate systems was summarized, and the reaction mechanism of the whole crystallization process was given. Research results: (1) In the three supersaturated solutions of Ascharite, the main polyborate ion is . After evaporation and crystallization of the three supersaturated solutions of Ascharite, Hungchaoite can be observed in the crystals, which is a stable species formed by the polymerization reaction of ; (2) The asymmetric stretching vibration of the three-coordinated boron-oxygen bond ν_as(B₍₃₎—O) in the magnesium borate crystal is mainly inward stretching. In contrast, the borate ions in the solution mainly stretch outward. The difference in vibration direction can be used as a basis to distinguish the presence of borate ions in crystals and solutions; (3) There is no in the magnesium borate salt obtained by the “dilution to salt” process, and in this study, only appears during the phase transition crystallization process of the supersaturated solution of Hungchaoite, which is explained by the fact that in the supersaturated solution of Hungchaoite is more likely to undergo polymerization reaction to generate . This study expands the traditional concentration range of borate system research. It describes the dynamic evolution process of magnesium borate in detail, which provides a new understanding of the infrared spectroscopy study of magnesium borate systems during phase change.