A dihydroxysilicate disulfide PEPA ester flame retardant, dihydroxy silicate bis-1-thiophospha -2,6,7- trioxabicyclo [2.2.2] octyl-4-methyl ester(SDSPE), was synthesized by silicon tetrachloride(STC) and 1-thiophospha-4-hydroxymethyl-2,6,7-trioxabicyclo [2.2.2] octane (SPEPA). The effects of solvent, reaction temperature, reaction time and mass ratio on the yield were investigated. The optimal conditions were selected as follows: dioxane as solvent, the first step kept the reaction going at 30℃for 6h when the molar ratio of STC and SPEPA was 1:2.1, and then reacted with two times amount of water at 60癈 for 4h to obtain a dihydroxysilicate disulfide PEPA ester. The yield could reach to 93.9%. The structure and flame retardance of the product were characterized by means of FTIR, 1H -NMR, TG-DTA analysis and LOI technique. Results showed that the product had a good thermal stability and high flame-retardant efficiency when applied to 191 unsaturated polyester.