MgAlON and α-Al2O3 have comparable optical refractive indices, and the composite ceramics composed of two materials are expected to obtain optimized microstructures, mechanical and thermal properties while maintaining reasonable optical properties. In this study, the optical transmittance behavior of MgAlON-Al2O3 composite ceramics were predicted based on Van-de-Hulst theory, and it is found that the theoretical in-line transmittance increases with the decrease of content and grain size of α-Al2O3 , and it can reach 86% at the wavelength of 2 μm. In the composite ceramics prepared by hot-press sintering, α-Al2O3 effectively inhibites the grain growth of MgAlON through grain boundary pinning effect. As the sintering temperature increases, α-Al2O3 content in the composite ceramics decreases. At the same time, the optical transmittance of ceramics increases, while the mechanical properties and thermal conductivity decrease. The maximum in-line transmittance of sintered samples with 1 mm thickness from 1 500 ℃ to 1 650 ℃ is greater than 72. 6% at a wavelength of 4 μm. Among them, 1 600 ℃ sintered sample shows the maximum in-line transmittance of 76. 1% , elastic modulus of 321. 69 GPa, hardness of 15. 93 GPa, four-point fracture strength of 268. 09 MPa, fracture toughness of 2. 41 MPa·m1 / 2 , and thermal conductivity of 13. 98 W·m - 1·K - 1 .