Due to the lack of control inputs, the motion range and application areas of underactuated mechanical system are greatly limited. To overcome such difficulties, the arbitrary state point dynamic tracking problem named dynamical servo control is discussed in this paper. Considering simplicity and repeatability, periodic trajectory is the best way leading to the desired point. Virtual constraints concept is employed and a target oriented trajectory planning procedure is proposed, which can generate periodic dynamic compatible and desired point crossing trajectory. A Lyapunov based tracking controller is proposed to avoid the strict conditions for using a traditional LQR based method. Experiments are performed on an Acrobot prototype and verify the correctness of the proposed periodic trajectory generating method and the effectiveness of the tracking controller.