Abstract: Metallic thermal protection structures used in high-Mach-number aircrafts face thermal matching issues in high-temperature operating environments. In this study, a sandwich panel structure with low thermal expansion properties is proposed to address this problem by adopting a concave orthogonal combination design of double materials in the core layer, which is connected to the panel through I-beams. The thermal deformation mechanism of the core unit of the structure is analyzed theoretically. The low thermal expansion properties of the structure are verified through numerical simulations and a comparison study with traditional honeycomb sandwich panels. Studied are the effects of design parameters such as the thickness of the beveled wall, the thickness of the web, the thickness of the straight wall, and the angle between the straight and beveled walls on the low thermal expansion performance. An optimization design method based on Bayesian optimization theory is proposed, where both equivalent areal density and structural safety are used as the optimization constraints, and the minimization of thermal deformation is set as the optimization objective. The structure is optimized upon these criteria. The results show that, compared to traditional honeycomb sandwich panels, the proposed structure exhibits better low thermal expansion performance. The optimization method proposed demonstrates a good convergence, and the structure optimized reduces the maximum thermal deformation by 28%, compared to the traditional honeycomb sandwich panel.