High-purity nano-crystal Mg(OH)2 powder was prepared, using self-made high-purity MgSO4 and mixed alkali solution of ammonia water and NaOH liquor as starting materials. The effects of several key factors on precipitation rate of Mg and average particle size of powder were studied. The results show that with increasing of initial concentration of Mg2+ and reaction temperature, the precipitation rate of Mg increases, while the average particle size of powder decreases at first, and then increases; if prolong the reaction time and ageing time, the precipitation rate of Mg and the average particle size of powder both increase; if increase stirring rate, the precipitation rate of Mg does not increase obviously, but the average particle size of powder decreases. When the initial concentration of Mg2+ is 2.0 mol×L-1, reaction temperature is 50 oC, reaction time is 60 min, stirring rate is 900 r×min-1, and ageing time 90 min, the precipitation rate of Mg is 95.4%; meanwhile the morphology of obtained Mg(OH)2 particles is hexagonal flake, the size of them is between 50 nm and 140 nm, the granularity distribution is uniform, the dispersibility is good, and the purity is high.