Given the imperative of energy conservation, safety, and durability in high-temperature industrial furnaces, current research focuses on reducing the thermal conductivity of refractories used in furnace linings while ensuring optimal performance. The research subject of this study was cement-bonded mullite castable. Firstly, the hydration behavior of calcium aluminate cement was regulated by incorporating magnesium silicate hydrate (M-S-H) with a high specific surface area to enhance bonding strength. Additionally, thermal conductivity was cooperatively reduced by lightweight mullite aggregate and introducing air-entraining agent in the matrix. The results indicate that M-S-H with a high specific surface area provides more nucleation sites for hydration products, thereby facilitating the formation of pin CAH₁₀ during cement hydration and improving castable bonding strength. Furthermore, through the incorporation of M-S-H and 0.01% (mass fraction) air-entraining agent in castables with lightweight mullite aggregate, the pore within 1~100 μm significantly increases after heat treatment at 1 250 ℃. Moreover, at 1 000 ℃, the thermal conductivity is only 0.599 W·(m·K)^-1, which is 18.9% lower than that observed in traditional castables.