Ceramic aerogels have promising applications in extreme environments, such as aerospace, but are severely limited by the sintering-prone nature of nanounits at high temperatures. However, they typically exhibit inadequate dimensional stability when exposed to high-temperature atmospheres, which can result in the deterioration of their macroscopic characteristics, eventually restricting their applications in extreme environments. Here, a (YYbErDyGd)₂SiO₅ ceramic aerogel (ESA) was prepared at 1050 °C by introducing high entropy into a ceramic aerogel, thereby enhancing its high-temperature thermal stability via the high-entropy effect. ESA was exposed to a temperature of 1600 °C for 2 h without undergoing sintering. In addition, different fiber-reinforced ceramic aerogel composites are also explored, with a thermal conductivity of only 0.032 W/(m·K) at room temperature and 0.108 W/(m·K) at high temperature of 1000 °C. These composites demonstrate no visible damage or deformation under extreme conditions across a substantial temperature range (−196 to 1300 °C), a property that is paramount for applications in extreme environments. At 1100–1300 °C, after 2 h of calcination, the composites exhibit a shrinkage rate of only 0.25%. After 600 s of butane torch ablation at 1300 °C, the back temperature is only 110 °C. Moreover, under 60% compression deformation, its maximum compression strength is 0.386 MPa. Even after 20 high-temperature thermal cycles (1300 °C for 2 h), the sample maintains a low thermal conductivity of 0.043 W/(m·K) and a compressive strength of 0.259 MPa. This work provides a new perspective for exploring the limits of the strength and thermal properties of ceramic composites in the field of high-temperature insulation, particularly under extreme conditions.