Lightweight of refractories is an important direction for high-temperature industry energy conservation. Lightweight M60 mullite materials were prepared via an in-situ decomposition pore-forming route using tertiary raw bauxite and fly ash as main raw materials. The effect of fly ash on the structure and properties of lightweight M60 mullite materials at different firing temperatures was studied. The results show that at the firing temperature of 1 400 and 1 500 ℃, the introduction of fly ash reduces the bulk density of materials, increases the total porosity of materials, and greatly increases the compressive strength of materials. The thermal conductivity of sample sintered at 1 500 ℃ decreases obviously. When the firing temperature reaches 1 600 ℃, the total porosity of material decreases, the bulk density increases slightly, and the thermal conductivity increases obviously. The introduction of fly ash also reduces load softening temperature of materials. After the introduction of 34.0% (mass fraction) fly ash, the bulk density of prepared mullite materials decreases by 8%~13%, and the total porosity increases by 23%~37%. After firing at 1 500 ℃, the compressive strength of materials reaches (122.6±2.2) MPa, and the thermal conductivity tested at 1 000 ℃ is only 0.616 W/(m·K).