Flame retardant magnesium hydroxide powder was prepared by hydrothermal modification method with magnesium hydroxide powder as raw material that was synthesized by precipitation of simulated concentrated seawater and sodium hydroxide. The effects of mineralizers and temperature on the morphology, crystalline structure and dispersion properties of hydrothermal products were examined. The magnesium hydroxide powder was characterized. The results show that alkali salts are superior to chlorine salts as mineralizers. In the experimental ranges, the higher the mineralizer concentration and temperature, the better the improved crystalline properties of product, such as more regular morphology, larger crystal size, greater plate thickness, less microstrain, superior crystallizability and dispersibility. Magnesium hydroxide particles as flame retardant with primary particle size about 0.250 mm, thickness of 61 nm and agglomeration number 10.95 can be obtained using 4.0 mol/L NaOH as mineralizer under the hydrothermal temperature of 200℃ and hydrothermal time of 8 h.