Abstract: The flame retardant type magnesium hydroxide has been prepared with brine，halide，and magnesium chloride hexahydrate as raw materials by two-step process (i.e.ammonia precipitation and hydrothermal treatment route).The effects of hydrothermal treatment， mineralizer and SO↑2-↓4 anion etc.on preparing flame retardant type Mg(OH)↓2 were compared and analysed.The results showed that compared with Mg(OH)↓2 products synthesized at normal temperature，the alteration of growth direction of Mg(OH)↓2 crystals after hydrothermal treatment results in the increase of I↓(001)/I↓(101) of the hydrothermal products which weakens the polarity of the Mg(OH)↓2 crystals.The counter-ion SO↑2-↓4 affects the structure of crystallites and the removal of impurity SO↑2-↓4 is favorable for the formation of the flame retardant type Mg(OH)↓2 with stable structure.The Mg(OH)↓2 products with regular-hexagonal plate，thin，high dispersion property and mean particle size of 200 nm could be obtained by two-step process.The analysis of chemical composition showed that the magnesium hydroxide has a high purity（98.92%~99.06%）with the quality coming up to the national standards.The two-step method has the advantages of accessible raw materials and low cost etc.And it could be easily used into industry production.