Abstract: Aerogel is a highly efficient thermal insulation material with significant potential for spacecraft thermal protection systems. However, at elevated temperatures, it exhibits high transparency to thermal radiation in the 3-8 μm wavelength band, resulting in enhanced radiative heat transfer. To suppress radiative heat transfer under high-temperature conditions, opacifiers are commonly incorporated into aerogel materials. In this study, a spectrally dependent coupled radiation-conduction numerical model was developed using the discrete ordinates method. The model was compared with the optically thick approximation model. The effects of SiC opacifier particle size and volume fraction on the effective thermal conductivity of silica aerogel composites were systematically investigated. The results showed that the optimal particle size was 2 μm over the temperature range from 300 K to 1400 K, with the minimum effective thermal conductivity of 0.03966 W/(m·K) achieved at 1100 K for a silica aerogel composite doped with SiC particles of 2 μm diameter at a volume fraction of 5%. This study provides theoretical insights and modeling support for the design and parameter optimization of opacifier-doped aerogel materials.