Abstract: High purity magnesium hydroxide was prepared by taking advantage of existing sodium hydrosulfide and brine from Yuncheng salt lake as raw materials.Through further hydrothermal modification treatment,flame-retardant magnesium hydroxide was obtained.Optimum conditions of hydrothermal modification were as follows:sodium hydroxide was hydrothermal modifier;concentration of NaOH was 4 mol/L;reaction temperature was 200 ℃;mass fraction of MgOH in hydrothermal system was 7.5%;reaction time was 5 h.Under optimum conditions,the obtained product met the requirement of flame-retardant magnesium hydroxide due to its narrow size-distribution range,small specific surface area,good dispersivity,and morphology of hexagon.