Taking alkaline magnesium sulfate whiskers and sodium hydroxide as raw materials, the proper process parameters were experimentally investigated for magnesium hydroxide whiskers preparation by hydro-thermal synthesis. The experimental results show that satisfactory product of magnesium hydroxide whiskers can be obtained under the proper synthesis conditions. The whiskers are characterized with smooth surface, straight morphology, and small diameter. The whisker product of magnesium hydroxide is of less than 0.5μm in diameter, around 100 in slenderness ratio, and 99.69% in purity under the condition of 4.0% initial concentration of alkaline magnesium sulfate whisker, stirring intensity of 100rpm, reaction temperature at 180°C, and reaction time of 4h. The synthetic mechanism of the whiskers was analyzed by the theories of anion coordination polyhedron nucleation and screw dislocation growth of the crystal.