No matter how complex the component of structures and connection area are, the static strength of civil aircraft is calculated and analyzed on the basis of elastic mechanics. The internal force solution of the natural mesh finite element model under the ultimate load case is 1.5 times liner analysis of the limit load case. In fact, most of the structures except for the special design requirements are allowed no harmful permanent deformation under limit load and no damage under ultimate load. If the structures is still linearly distributed under ultimate load case, it can only show the low efficiency of structure, and the competitiveness and advancement of the commercial aircraft can not be mentioned. So what are the conditions and the premises for this linear hypothesis? This paper points out the premise of the hypothesis through the concept of the structural shakedown.In this paper, new concept of structural design and strength analysis method for civil aircraft are put forward, that are shakedown of structure. Through the interpretation of structural stability, the interpretation of CCAR25.303 and 25.305(a) regulations are extended to provide support for aircraft type certification.