The flame retardant mechanism of magnesium hydroxide sulfate whiskers(MHSH) is analysed and MHSH and magnesium hydroxide are used to fill high-density polyethylene(HDPE) respectively. The mechanical and flame resistance properties of composite material (HDPE) were also studied in this paper contrastively. The tensile strength and elongation of the compound are increased 2.5% and 36% severally, while MHSH content in weight reaches 20%;at the same time, the oxygen index (OI) of composite materials varies from 17 to 24.2. In contrast, the tensile strength and elongation of the HDPE/ Mg(OH)2 compound are decreased 6.4% and 48% respectively, OI being 19.6. To sum up, the results of tensile and flame tests of compound material showed the HDPE containing as-prepared whiskers had better overall properties，exceeding that of Mg(OH)2.