Polysiloxane (PSO) was adopted as the matrix of the repair agents, and SiC-ZrB₂ powder was used as the filler, to repair the prefabricated defects on the SiC-ZrB₂/SiC (SZS) coating of carbon/carbon (C/C) composites. The repair agents were brushed on the defect areas and then underwent preoxidation (PR) or heat-treatment (HR) in a vacuum. The effects of different treatment processes on the chemical composition, microstructure of the repair agents, and the oxidation resistance behavior of the repaired coating were investigated. The repaired agents after both processes were pyrolyzed and generated SiOC ceramics, and they were well combined with the original coating. The thermal stability of PSO after preoxidation is poorer than that after heat-treatment, resulting in a weight loss rate of 5.88% after oxidation at 1500 ℃ for 270 min, while that of the HR coating is only −0.87%, yet both have been great improvement compared with the unrepaired coating. This work provides an effective and simple approach to repairing damaged coatings for high-temperature applications.