The invention provides a method for preparing low-surface activity and large-grain size magnesium hydroxide. The method comprises the following steps of: 1, adding a small amount of lime milk and a calcium chloride solution into brine, stirring to form a small quantity of Mg(OH)2 flocculent precipitates, filtering the precipitates, and removing SO4<2-> and other impurities in the brine; 2, adding calcium chloride in the brine from which the impurities are removed, so that the brine contains Ca<2+> of certain medium concentration; 3, adding the lime milk into the brine containing Ca<2+> of certain medium concentration, and after reaction is finished, precipitating magnesium hydroxide crystallite precipitates; and 4, settling the magnesium hydroxide crystallite precipitates, filtering, washing and drying.