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Silica aerogel composite is an excellent thermal insulator for spacecraft under high-temperature and complex air environments. This study intends to evaluate SiC-doped silica aerogel’s thermal insulation performance under large temperature and air pressure differences. In this paper, the hot surface’s temperature response of SiC-doped silica aerogel with different content was studied at significant temperature differences (Δ<i>T</i>) when pressure changes instantaneously. Their thermal insulation performance was evaluated by analyzing the influence of pressure gradients on the unsteady-state heat transfer. When the cold surface’s temperature of the specimen keeps constant at 15 °C and Δ<i>T</i> = 171~912 K, the results demonstrate that the correlative thermal conductivities of silica aerogel with 1% and 5.84% SiC are 0.02223~0.04077 W·m⁻¹·K⁻¹ at <i>P</i> ≈ 10 Pa and 0.03165~0.04665 W·m⁻¹·K⁻¹ at <i>P</i> = 1 atm, respectively. The aerogel composite with 0% SiC showed the best thermal insulation performance at Δ<i>T</i> < 200 K and <i>P</i> ≈ 10 Pa, while the aerogel with 5.84% SiC became the best at Δ<i>T</i> > 700 K and <i>P</i> = 1 atm. In addition, the transient pressure decreases will significantly impair the heat transfer of the gas inside the aerogel, thereby weakening the gaseous thermal conductivity and improving the thermal insulation performance.