The motion control of robot manipulator is inseparable from the manipulator dynamics. However, the relevant literature often discusses dynamics model and control method separately. This paper attempts to design a good and feasible dynamics model to enhance the motion control of manipulator. Firstly, a dynamics model of the flexible manipulator was proposed through finite matrix analysis, based on the Lagrange's dynamics equations under different control conditions. Based on the flexible dynamics model, two adaptive control systems were designed based on whether the interaction force is known. In the control stage, a separated controller was developed by applying the adaptive model and sliding mode principle. The stability and robustness of our control strategy were proved through Lyapunov's method. Finally, several numerical tests were carried out the verify the feasibility and superiority of our control strategy. The research results provide new insights into the dynamic control of robots with varied load constraints and any degree of freedom (DOF).