Recently the authors had successfully applied the strip theory to predict the motion of the SWATH ship running freely in waves by frequency domain analysis. In the present paper, a time domain simulation technique was extended to evaluate the motion response of a SWATH ship with fixed fin in waves. The nonlinear effect aroused from the large motion and the nonlinear viscous force is also taken into consideration. The surge effect on pitch moment is also included to make the motion prediction more accurate. The time history for the equations of motions are solved by 4th Runge Kutta method. The corresponding hydrodynamic coefficients at each time step are calculated by applying an auto-mesh generation technique which needs only one set of initial input of the ship configuration. To verify the validity of the theoretical prediction, a series of model tests were handled in NCKU-DNAME towing tank for comparison. The results show that the technique developed here can be applied to predict the time domain simulation of SWATH ship motion with fixed fin in large longitudinal waves. Hopefully it can be further extended to study the controllable fin case incorporating with the advanced control techniques.