In order to analyze the influence of the forced vibration to the dynamic responses of the multi-body system with a floating base, the system is simplified to two rigid-bodies model connected by a smooth joint. The dynamic responses of the system are solved by the program according to the transfer matrix theory of multi-body dynamics. The time histories of the motion of the system are obtained under the condition of the floating base acted by forced period rolling motion or waves. The results of numerical simulation show that the swing of the weight severe with the increase of the amplitude and the frequency of the forced vibration, but not be influenced by the length variation of the rope. The oscillation amplitudes of the floating base and the weight under the action of the transverse regular wave increase with the length of the rope and the elevation of the arm.