Abstract: Until now, there have been calculation formulations for added water mass of monohull and eatamaran in the research of ship overall vibration. But for the overall vibration of muhihull, such as trimaran, there has bee n little research domestically and overseas. With the increasing need of trimaran, its overall vibration should be calculated in the process of design. And the proper calculation of its overall vibration is in need of its added water mass. A simple method for calculating the added water mass of ternate cylinders by means of the CFD software Fluent is put forward in the paper. From the calculation, the change of their added water mas s with their distance is obtained, and the added water mass of unit length of the trimaran vertical vibration is also obtained, which provides foundation for the trimaran overall vibration.