Abstract: Reverse precipitation method was applied to synthesize hexagonal flaky magnesium hydroxide by using brine of salt lake and ammonia as raw materials. The effects of magnesium ion concentration, reaction temperature, feed rate and aging time on the crystallinity, particle size and chloride content of magnesium hydroxide were studied, and the crystal plane growth of magnesium hydroxide was comprehensively reflected. Herein, characterization analysis of the samples was carried out by SEM, XRD and laser particle size analyzer. The results showed that, with the increase of Mg²⁺ ion concentration and reaction temperature, it was favorable for the crystal plane growth of magnesium hydroxide(001). The increase of Mg²⁺ ion concentration and the prolongation of aging time made the chloride content of magnesium hydroxide increase. Its particle size was greatly affected by reaction conditions, crystal plane growth and polarity. Hexagonal flaky magnesium hydroxide with (001) crystal face size of 13.1-19.4 nm, particle size D₅₀ of 1.64-9.32 μm, and chloride content < 0.1% could be prepared by brine-ammonia reverse precipitation. These results could not only provide a scientific basis for regulating the growth of magnesium hydroxide crystal planes prepared from bischofite in salt lakes, but also offer a useful reference for using salt lake resources to prepare high-purity, high-value-added functional materials of magnesium hydroxide.