A novel high-molecular-weight sulfur-bearing secondary antioxidant, OVPOSS@MB, was synthesized by thermal-initiated thiol-ene click reaction of 2-mercaptobenzimidazole (MB) with octavinylsesquisiloxane (OVPOSS). The chemical structure of OVPOSS@MB was confirmed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, and the effects of OVPOSS@MB on the vulcanization characteristics, tensile properties, and oxidation resistance of EPDM were investigated. The results show that OVPOSS@MB slows down the vulcanization rate and reduces the degree of cross-linking of rubber, which makes the OVPOSS@MB/EPDM composites exhibit lower rubber modulus and higher elongation at break. In addition, compared with the low-molecular-weight antioxidant MB, the high-molecular-weight antioxidant OVPOSS@MB has higher anti-migration properties, together with the free radical scavenging function of the OVPOSS lattice, making OVPOSS@MB have more excellent antioxidation performance than MB.