Abstract: This study prepared mullite-corundum composite ceramic shells using silica sol as a binder with white corundum and mullite as refractory powders. Different gradient final firing temperatures were employed to investigate their effects on shell performance, phase transformations, and fracture behavior, elucidating the relationship between phase evolution and performance enhancement. Results demonstrate that the final firing temperature significantly governs the phase evolution of Al₂O₃ and 3Al₂O₃·2SiO₂ (mullite). During 1050–1200℃, SiO₂ and Al₂O₃ form an Al-Si spinel-type phase while pre-existing 3Al₂O₃·2SiO₂ decomposes to generate the same Al-Si spinel phase. Above 1200℃, γ-Al₂O₃ transforms into stable α-Al₂O₃, with SiO₂ partially undergoing polymorphic transition to cristobalite while the remaining portion reacts with α-Al₂O₃ and Al-Si spinel phase through secondary mullitization to form 3Al₂O₃·2SiO₂ phase. With increasing final firing temperature, the fracture mode shifts from intergranular fracture (characteristic of single-phase alumina) to mixed transgranular-intergranular fracture. The synergistic effects of secondary mullitization and cristobalite formation substantially enhance flexural strength, peaking at 29.94 MPa at 1350℃.