<p>It is important to enhance the mechanical properties and thermal shock resistance for the application of low thermal conductivity mullite castable in harsh service environments. The special hollow mullite aggregates with silica-rich glass phase were firstly fabricated by a templating method. The mullite castables containing both traditional dense and synthesized hollow mullite aggregates were prepared, and their thermal conductivity, mechanical properties, and thermal shock resistance were investigated. Compared with the mullite castable with traditional aggregates, the thermal conductivity of the castable with hollow aggregates decreased, and their mechanical properties and thermal shock resistance were enhanced evidently. At elevated temperatures, the silica-rich glass phase in the hollow aggregates reacted with Al₂O₃ in the matrix to form in-situ mullite, thereby altering the bonding mechanism at the aggregate/matrix interface and enhancing the specific fracture energy of the material. When 50% of the traditional mullite aggregates were replaced by hollow aggregates, the room-temperature flexural strength and specific fracture energy of the mullite castable increased by 21.8% and 112.1%,&#xa0;respectively, while the thermal conductivity at 1000 °C was reduced to as low as 0.859 W/(m K).</p>