Abstract: In the absence of surfactant，magnesium hydroxide flame retardant was prepared，with light burning powder refined liquid after eliminating sulfate as raw material，and ammonia as the precipitator.The as-prepared samples were characterized by X-ray diffraction（XRD），scanning electron microscopy（SEM），and laser particle size analyzer.The effects of various operating variables，including the ammonia flow rate and reaction temperature，on the morphology and particle size of magnesium hydroxide were investigated. Under the optimum reaction temperature and the ammonia flow rate，the influence of pure seed crystal addition amounts on the product was also investigated.Results showed that pure seed addition amount of 3%（mass fraction），ammonia flow rate of 300 mL/min，and the temperature of 90 ℃，were the best production conditions.Under those conditions，the product′s particle size D50=1.2 μm，specific surface area（BET） was 6.3 m2/g，and conversion rate reached 81.2%.

Key words: light burning powder, flame retardant, magnesium hydroxide