<p>This paper focuses on the rigid-flexible coupling dynamic characteristics of spacecraft with flexible structures of different lengths, especially the dynamic stiffening phenomenon and its influence on the overall performance. Different length flexible structures are abstracted into different physical models, and modeling is carried out based on the characteristics of the physical models. The aim is to construct more accurate physical models, unconstrained mode solving methods, and reveal the factors that cause dynamic stiffening phenomenon. The slender beam is considered as Euler beam, and the longitudinal shortening caused by lateral deformation is taken into account during the modeling process. Unconstrained mode and unconstrained mode orthogonality are used for modal solution. Consider the thick and short beams as Timoshenko beams, and take into account the effects of centrifugal force and shear deformation during the modeling process. The Frobenius method is used to solve their unconstrained mode. Finally, given the spacecraft speed, the beam end response was obtained through MATLAB numerical simulation. The simulation results showed that the unconstrained mode modeling method does not affect the occurrence of dynamic stiffening phenomenon, but only affects the response accuracy. The lateral deformation of a beam can lead to longitudinal shortening, and considering the quadratic coupling term of the lateral and longitudinal deformation of the beam during the modeling process results in dynamic stiffening. This indicates that the longitudinal deformation of the beam will affect the generalized stiffness of the model.</p>