Synthesis of high-purity magnesium hydroxide using dolomite and bittern is important for use in various applications. We synthesized magnesium hydroxide using bittern and dolomite, which are domestic resources. In Bittern, there is a high concentration of Mg2+ ions, but the impurity Ca2+ ion content is also significant, requiring a purification process to remove it. There are two main methods for this purification. Firstly, there is a separation method that utilizes the difference in solubility between Mg2+ ions and Ca2+ ions by using sulfuric acid on dolomite. Adding MgSO4 solution from dolomite to Bittern removes Ca2+ ions as CaSO4. This process simultaneously purifies Ca impurities and increases the Mg/Ca ratio by adding extra Mg2+ ions. In this study, purified bittern was obtained by using dolomite and sulfuric acid to extract MgSO4, which was then used to purify Ca2+ ions. High-purity Mg(OH)2 was synthesized by optimizing the NaOH and NH4OH ratio as an alkaline precipitant. Mg(OH)2 synthesis technology made by effectively removing Ca ions from dolomite and bittern can contribute to domestic pilot production.