The surface modification of soy protein isolate (SPI) and the use of modified SPI as reinforcing filler replaced carbon black in BR/SBR composites were studied. The effects of coupling agents gamma-aminopropyltriethoxysilane (KH-550), gamma-methacryloxypropyltrimethoxysilane (KH-570), bis-(gamma-triethoxysilylpropyl)-tetrasulfide (Si-69), isopropyl tri-(dioctylpyrophosphate) titanate (NDZ-201) on the physical and mechanical properties of the composites were also investigated. Modified SPI was analyzed by Infrared spectra, scanning electron microscopy, and X-ray diffraction. The results showed that the optimal reaction conditions were determined, that is, the optimal reactions were 4 g SPI, 50% Glycidyl methacrylate (by SPI weight), 3 h of reaction time and 80 A degrees C reaction temperature. Through the physical and mechanical properties of composite, the results showed that when a small number of modified SPI (3-5 phr) as reinforcement agent was used, the mechanical properties were improved. KH-550 had good effect on enhancing the overall properties of the composites.