The flexible-link flexible-joint ( FLFJ) manipulator is typically a nonlinear, strong coupling, and under-actuated system being quite difficult to be well controlled. To improve control performance, proper selection of system's dynamic model is of utmost importance. In this paper, the dynamic modeling problem of FLFJ manipulators is studied, and a modified modeling method is proposed. In this method, the flexible joint connecting a flexible link is no longer simplified as a rigid joint and a torsion spring, but simplified as a rigid joint and an elastic constraint of the flexible link. Then, according to structural dynamics, Hamilton, s principle, and the Assumed Modes Method, the rigid-flexible coupling dynamic equation is established. Compared to the traditional method, our proposal can reduce the degrees of freedom of the system, and establish a more proper dynamic model for controller design. At last, numerical simulations verify the proposed dynamic model.