Abstract: A model is established based on the basic mechanics characters of composite materials and the basic mechanics theory of the multi - spar st ructures , combining the request from engineering practices. The equations ofloading capability are deduced , especially in post-buckling of the multi-spar st ructures , and the analyzing sof twareis designed. The validity and effectiveness of the sof tware are demonst rated by comparing the application with experiment . The sof tware with high accuracy and short-time-consumption can solve the analytical problem of composite multi-spar load capability faced in the aircraf t design.