Soy protein has been considered as a kind of natural polymer and an ecological material that was widely used in many fields such as food industry. In this paper, the soy protein isolate (SPI) was modified by glutaric dialdehyde and characterized by IR spectra, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The modified soy protein isolate as reinforcing filler partly replaced some carbon black was used to prepare the SPI/rubber composites. Based on the mechanical and dynamic tests of their vulcanizates, the results showed that when a small amount of modified SPI (8~10phr) as reinforcement agent was used, the mechanical properties were improved. Moreover, the changes of their lgE’ and tanδ occurred.