In the field of marine construction, traffic ships are used to board the floating structures. However, the position of a traffic ship can be fluctuated because of tidal waves, and workers face the risk of an accident such as falling into the water or a collision with the vessel. Due to such potential hazards, we focus our attention on the safety and workability of such ship fluctuations. In this study, an active vibration compensator with a Stewart platform has been proposed and developed. The platform is supported on the main hull by means of a six-degrees-of-freedom hydraulic parallel mechanism that absorbs the motion of the main hull in accordance with the control signal from an on-board computer and motion sensors. Results of the simulation models confirm the design of the motion range that is required for the actual active vibration compensation system. Trial results depict that 66%-84% of the heave, roll, and pitch motion of the main hull is absorbed.