Abstract: According to the problem of a trimaran cross-deck structure's fatigue strength, taking one trimaran as an example, the responses of ship motion and hydrodynamic pressure on the ship's surface in regular waves are calculated on the basis of the 3D linear potential flow theory. Next, the stress responses of hot-spots in regular waves with different wave angles are evaluated by the finite element analysis of the global trimaran structure. On the basis of linear cumulative damage theories, the fatigue damage is calculated according to the direct calculate method using spectral analysis. Finally, the effect of different sea areas and heading angles' time factors are discussed. The results show that the fatigue damage is greater when using an inshore sea area and considering the time factors of heading angles. The result can offer a reference for the design and development of trimarans.