An innovative design of C/SiC sandwich composites with graded lattice cores was proposed for the purpose of thermal protection system of high-speed aircraft. Considering the coupling of heat conduction，heat radiation and heat convection，three-dimensional heat transfer model of the sandwich structure was established based on Discrete Ordinates Ｒadiation Model and Laminar Flow Model. The calculation method for heat transfer coefficient of the graded lattice core sandwich structure was presented. In order to verify the calculation method， the numerical results were compared with the experiment data. The influences of temperature boundary conditions and meso size of the sandwich structure on the heat transfer coefficient were investigated. The key factors to design C/SiC graded lattice core composite sandwich structures were presented.