The third mirror mantle is a carrier for affixing calefaction patch. First of all, confirm the distance between the third mirror mantle and the third mirror considering thermodynamics. Then, design and optimize the material and structure project of the third mirror mantle. Finally, analyze statics, dynamics and thermal characteristics using Finite Element Method (FEM). The result indicates that the project meets the statics and dynamics requirements of design and application, and the project is reasonable and feasible.