Abstract: A new preparation technology for nano-sized magnesium hydroxide has been developed which takes the natural brine from salt lakes as raw material，lime milk as precipitator，adopts special calcium-removing technique and controls the temperature at 40～90 ℃ during the reaction.With this technology，the Mg(OH)↓2 content of the product was more than 98%，the CaO content was less than 0.2%，decomposition temperature was 396.93 ℃ and the thermal mass loss was 30.96%.The topography of the product has been characterized by SEM.Results showed that the mean particle size of the magnesium hydroxide was about 80 nm and the particles were lamellar grains.The specific surface area was 39.2 m2/g.This technology has the advantages of simple process and low cost and it is a cleaner synthesis route.The quality of the product not only reaches the National Standard，but also gets to the foreign enterprise standard.Therefore，the industrial production was being expected.