With γ-chloropropyltriethoxysilane (γ2),sulfur and sodium disulfide as the starting materials，the bis-(triethoxysilypropyl)disulfide (Si-75) was synthesized in aqueous phase by using phase transfer catalysis technique and adding buffer to the reaction system to control its pH value which can refrain the hydrolysis of γ2. In this paper, the mol ratio of sulfur with sodium disulfide, γ2 with sodium polysulfide and sodium carbonate with sodium disulfide, reaction temperature, the mass of phase transfer catalyst and some other conditions are all studied. Using this method, we got the sample with yield of 95.4%，sulfur content of 13.5% and flaxen appearance .