A caged phosphorothioate flame retardant synergistic with silicon, phosphorus and sulfur three elements, diphenyl silicic acid bis-1-phosphorus heteroaryl thio-2,6,7-trimethyl- oxabicyclo [2.2.2] octyl-4-methyl ester, was synthesized by dimethoxydiphenylsilane (DMDPS) and 1- phosphorus heteroaryl thio-4-hydroxymethyl-yl-2,6,7-trimethyl- oxabicyclo [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: n(DMDPS): n(SPEPA) is 1:2.2, kept the reaction going at 140℃ , then fractionating for 7h and the yield can reach to 92.7%.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 have a good thermal stability and high flame-retardant and anti-dripping efficiency for polybutylece terephthalate (PBT). Besides, when it was mixed with MPP and was applied to PBT, high synergistic flame-retardance efficiency was found.