Abstract: The superfine flaky magnesium hydroxide powder was prepared by direct precipitation method，with boron slurry as magnesium source and ammonia as precipitator.Through study on the influences of different reaction conditions on the production rate of magnesium hydroxide，the optimum technological conditions for synthesis of magnesium hydroxide were eventually obtained.The product was characterized by XRD，TG，SEM，and particle-size analyzer.The results showed that the product of the experiment was ultrafine magnesium hydroxide，the crystal was well structured，and the morphology of which was six-party flake.The optimum synthetic process conditions were as follows：leaching with hydrochloric acid at room temperature to get magnesium solution，pH was 11，reaction temperature was 60 ℃，subsidence time was 2 h，ammonia dilution ratio （volume ratio） was 1∶1，and dropping speed of ammonia was 1 drop/s.Adding anhydrous ethanol can effectively improve the binding and dispersity of magnesium hydroxide.

Key words: boron slurry, magnesium hydroxide, productivity, particle size, morphology