A kind of magnesium oxychloride crystal salt with high content of 5•1•8 crystalline phase was prepared in this research. And detailed experimental studies were carried out to determine the effect of magnesium oxychloride crystal salt on the flame retardance and thermal properties of compression molded SMC parts. Results showed that the comprehensive properties of SMC were optimum when the weight content of magnesium oxychloride crystal salt was 40%, and the oxygen index was 35.6 and the inflaming level was V-0 according to UL-94. The thermal decomposition process of pure magnesium oxychloride crystal salt was carried out in the following three steps: releasing crystal water, then HCl secondly and finally structural water, the residua was magnesia. The apparent activation energy of reaction in each step was 124.31kJ/mol, 111.50kJ /mol, 113.40 kJ /mol and the reaction order was 2.11, 2.04, and 0.98 respectively.