High purity ultrafine flame retardant magnesium hydroxide powder was prepared by the liquid phase precipitation method using magnesium sulfate as raw material, sodium hydroxide as precipitant, and sodium stearate (NaSt) and polyethylene glycol (PEG2000) as surfactants. The effects of precipitation mode, reactant ratio, reaction temperature, stirring rate, reaction time, surfactant ratio, surfactant dosage, and washing method on the particle size of magnesium hydroxide powder were investigated. The product morphology, crystalline phase structure, particle size and its distribution, purity, dispersion effect, and wettability were tested. The results show that the suitable process conditions for the preparation of ultrafine magnesium hydroxide are as follows: the precipitation method is two-way precipitation, n(NaOH)∶n(MgSO₄)=2.5∶1.0, the reaction temperature is 60 ℃, the stirring rate is 400 r/min, the reaction time is 50 min, the ratio of NaSt/PEG2000 is 4∶1, the addition amount is 4 g/mol MgSO₄, and the washing method is pure water washing for three times. The surface of the obtained magnesium hydroxide is hydrophobic, the contact angle is up to 123.50°, the dispersion is good, the purity is as high as 99.50%, the average particle size is 3.83 μm, and the particle size distribution is uniform and hexagonal lamellar crystal. The whiteness, purity and particle size of the sample met the magnesium hydroxide index for flame retardant.