Structural instability failure is common in aircraft static test，and structural stability failure will lead to structural failure of the whole aircraft. The opening of the composite material rear spar put forward higher requirements on the stability of structure. Firstly we have to meet the static strength requirements, then the stability of the structure will be made an optimization design. Based on theoretical calculation and finite element analysis software Nastran, this article analyzes the rest rear spar stability and makes columns of rear spar rest structure layout optimization design and section size optimization design. It will provide detailed design parameters support for the composite material rear spar.