Abstract: Lightweight mullite-corundum castables were prepared using porous lightweight mullite aggregate (5~15 mm、1-≤5 mm and ≤1 mm), tabular corundum fine powder (≤0.074 mm), mullite fine powder (≤0.074 mm), Guangxi white clay (≤0.045 mm), and CA-70 cement as main raw materials, with the addition of expansive agent kyanite of different particle sizes (0.425 mm, 0.320 mm, 0.15 mm, 0.075 mm, 0.045 mm) . The effect of kyanite particle size on the microstructure and properties of the castables was then investigated. The results show that after calculated at 1 400 ℃ , the particle size of kyanite significantly influences the mullitization behavior. As the particle size decreases, the mullitization reaction becomes more complete, the permanent linear change rate decreases markedly, and the microstructure becomes more uniform and dense. The bulk density, cold modulus of rupture (CMOR), and cold crushing strength (CCS) of the specimens all first increase and then decrease, while the thermal shock resistance improves. Among all specimens, the one with 0.075 mm kyanite (M200) exhibits the optimal performance with a bulk density of 1.76 g/cm³, CMOR of 9.8 MPa, and CCS of 64.7 MPa, which are 6.5% and 28.9% higher than those of the M35 specimen, respectively. Its permanent linear change rate is only 0.03%, a reduction of 94.6%, the residual strength retention rate after thermal shock reaches is 33.3%, and the thermal conductivity at 300°C is 0.32 W/(m•K). An appropriate amount of fine kyanite particles optimizes the particle size distribution and promotes the whiskerization of mullite and the formation of its interwoven network structure, synergistically enhancing the castable’s high-temperature volumetric stability, thermal shock resistance, and thermal insulation capability. The insulation cover made of this castable remains structurally intact after 648 service cycles at Baosteel, and its service life exceeds the design requirement, providing reliable support for the lightweight design, high stability, and long service life of torpedo ladle car insulation covers.