The dynamics modeling of the mechanical system with flexible deformation and rigid body motion was discussed. Regarding the rigid body motion degree of freedom and the elastic deformation degree of freedom as the generalized coordinate,and using the finite element method to describe the motion and deformation of the elastic connecting rod with elastic deformation and rigid body motion,we obtained the generalized inertial force in terms of the rigid body displacement and the elastic deformation displacement.Considering the relationship of the stressstrain,we also obtained the structural stiffness matrix,which represents the elastic deformation,and the geometric nonlinear stiffness matrix,which represents the nonlinear deformation of the deformed body.Using the Kane equation,we derived the movement equation of the elastic connecting rod organization. This kind of modeling method can be used in the mechanical system with arbitrary structure.