A modified polysulfide rubber terminated with alkoxy silane was prepared by termination reaction, using γ-(2,3-epoxy propoxy) propyl trimethoxysilane (KH-560) as the end-capping modifier of liquid polysulfide rubber. The conversion rate of thiol group in the reaction was measured by chemical titration. The structure of modified polysulfide rubber was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (1H-NMR). The molecular weight of modified polysulfide rubber was analyzed by GPC. The results show that the epoxy group of KH-560 reacts with the thiol group of liquid polysulfide rubber. The conversion rate of thiol group can be controlled by adjusting the ratio of reactants. The number-average molecular weight (Mn) of modified polysulfide rubber is 3641, and the polydispersity index (PDI) is 1.578. The modified polysulfide rubber could be used as the raw rubber of one-component elastic polysulfide sealants.