The invention discloses a dynamics modeling and analyzing method for an aerospace vehicle and belongs to the technical field of flight dynamics modeling and simulation analysis of aircrafts. According to the dynamics modeling and analyzing method, the variable-span and variable-sweepback aerospace vehicle is simplified and is composed of a fuselage, a left wing inner side, a left wing outer side, a right wing inner side and a right wing outer side, wherein the fuselage, the left wing inner side, the left wing outer side, the right wing inner side and the right wing outer side are independent rigid bodies. The deformation movement of wings relative to the fuselage is represented through a set of constraint equations, so that the freedom degree of a dynamics model is reduced; the aerodynamic force exerted on the aerospace vehicle is simplified to act on the fuselage only, and the dynamics model is established through the Kane method; related items derived from the deformation movement are extracted from the dynamics model to form additional force and additional force moment so that the influence on the dynamics characteristics of the aerospace vehicle by the inertia force and inertia moment caused by the deformation moment can be represented. The dynamics model of the aerospace vehicle is simplified through the dynamics modeling and analyzing method, and integral items and derivation terms of the inertia moment do not appear in the equations. In this way, dynamics modeling and simulating can be easily performed on the aerospace vehicle, and meanwhile accuracy is high.