The invention discloses a preparation method of a magnesium hydroxide flame retardant and the magnesium hydroxide flame retardant. The preparation method comprises the following steps: (1) preparing a magnesium chloride solution, wherein mass percentage of magnesium chloride in the magnesium chloride solution is 20-40%; (2) heating the magnesium chloride solution to required temperature and adding a composite precipitator, which is formed by mixing calcium oxide, magnesium oxide, sodium hydroxide and water, to the solution; (3) after the composite precipitator is added completely, washing and filtering the precipitate to obtain magnesium hydroxide filter cake, and then repulping the magnesium hydroxide filter cake; (4) drying the magnesium hydroxide filter cake in a vacuum drying box at 50 DEG C for 3-10 h; and (5) crushing and grinding the dried magnesium hydroxide to produce the magnesium hydroxide flame retardant. Purity of magnesium hydroxide in the flame retardant reaches not less than 99.0%. The flame retardant has excellent flame retarding effects and has simple preparation process and low cost, and is suitable for industrial production.