The invention discloses a method for preparing high-purity hexagonal piece-shaped magnesium hydroxide from magnesium oxide. The method comprises the following steps of: (1) adding distilled water into a reactor, heating to 60-80 DEG C, adding magnesium oxide in a solid-to-liquid ratio of 1g to 8g-10g, stirring for hydration reaction, carrying out suction filtration and solid washing, and drying, so as to obtain magnesium hydroxide; and (2) heating a magnesium chloride water solution to 60-70 DEG C, adding 5g of magnesium hydroxide obtained in the step (1), stirring, carrying out suction filtration, washing, and drying, so as to obtain high-purity hexagonal piece-shaped magnesium hydroxide. According to the method, magnesium oxide is prepared from magnesium oxide, the obtained magnesium oxide product has the advantages that the purity is high, the size distribution is good, crystals s are hexagonal piece-shaped and the like; a high-temperature hydrothermal modification procedure is omitted, and the prepared magnesium oxide product can be used as a flame retardant, so that the production cost is greatly lowered, and particularly, a novel way is provided for the development and utilization of the magnesium resource in western saline lakes; the method is an environment-friendly technique.