Magnesium hydroxide was converted between 340℃ and 490℃, and the endothermic energy was 187cal/g. The average temperature of the initial conversion of magnesium hydroxide is much higher than that of alumina hydrate, and it has good heat stability, excellent flame retardant grade and smoke removal effect. It is very suitable for the production and processing of polyolefin plastic products with higher average temperature. When Mg(OH)2 is applied to PP (more than 50%), it has excellent flame retardant grade efficacy. When Mg(OH)2 is applied to PE, its flame retardant grade efficacy is stronger than AI(OH)3. This is because magnesium hydroxide when ignited not only achieve dehydration chemical reaction, but also have a corresponding carbonization effect on the polymer to produce a protective layer, with flame retardant grade effect.