In order to achieve the goal of high value utilization of dolomite resources,the flame retardant Mg(OH)₂ was prepared from light-burned dolomite powder by the method of ammonia evaporation-NH₃ precipitation.The influences of various conditions on the product indexes and reaction mechanisms during the preparation of  Mg(OH)₂ by ammonia evaporation and NH₃ precipitation were studied.The results showed that the refined liquid with Mg²⁺ conversion rate of 81.5% could be obtained under the optimized process conditions of reaction temperature of 100 ℃,reaction time of 3 h and ammonium magnesium ratio of 2.2∶1.The flame retardant Mg(OH)₂ could be prepared by adding NH₃ into the refining solution when the reaction temperature was 50 ℃.The mechanisms of preparing flame retardant Mg(OH)₂ was elucidated by Materials Studio molecular simulation software.